Create an enum that allows us to distinguish between different
hashing algorithms. This enum is embedded into each
`git_hash_ctx` and will instruct the code to which hashing
function the particular request shall be dispatched.

As we do not yet have multiple hashing algorithms, we simply
initialize the hash algorithm to always be SHA1. At a later
point, we will have to extend the `git_hash_init_ctx` function to
get as parameter which algorithm shall be used.

Create a separate `git_hash_sha1_ctx` structure that is specific
to the SHA1 implementation and move all SHA1 functions over to
use that one instead of the generic `git_hash_ctx`. The
`git_hash_ctx` for now simply has a union containing this single
SHA1 implementation, only, without any mechanism to distinguish
between different algortihms.

As a preparatory step to allow multiple hashing APIs to exist at
the same time, split the hashing functions into one layer for generic
hashing and one layer for SHA1-specific hashing. Right now, this is
simply an additional indirection layer that doesn't yet serve any
purpose. In the future, the generic API will be extended to allow for
choosing which hash to use, though, by simply passing an enum to the
hash context initialization function. This is necessary as a first step
to be ready for Git's move to SHA256.

As we will include additional hash algorithms in the future due
to upstream git discussing a move away from SHA1, we should
accomodate for that and prepare for the move. As a first step,
move all SHA1 implementations into a common subdirectory.

Also, create a SHA1-specific header file that lives inside the
hash folder. This header will contain the SHA1-specific header
includes, function declarations and the SHA1 context structure.

The hash source files have circular include dependencies right
now, which shows by our broken generic hash implementation. The
"hash.h" header declares two functions and the `git_hash_ctx`
typedef before actually including the hash backend header and can
only declare the remaining hash functions after the include due
to possibly static function declarations inside of the
implementation includes.

Let's break this cycle and help maintainability by creating a
real implementation file for each of the hash implementations.
Instead of relying on the exact include order, we now especially
avoid the use of `GIT_INLINE` for function declarations.

The structure `git_hash_prov` is only ever used by the Win32 SHA1
backend. As such, it doesn't make much sense to expose it via the
generic "hash.h" header, as it is an implementation detail of the Win32
backend only. Move the typedef of `git_hash_prov` into
"hash/sha1/win32.h" to fix this.

largefile tests: only write 2GB on 32-bit platforms

Don't try to feed 4 GB of data to APIs that only take a `size_t` on
32-bit platforms.

Fix broken link in README

net: remove unused `git_headlist_cb`

cmake: default NTLM client to off if no HTTPS support

If building libgit2 with `-DUSE_HTTPS=NO`, then CMake will
generate an error complaining that there's no usable HTTPS
backend for NTLM. In fact, it doesn't make sense to support NTLM
when we don't support HTTPS. So let's should just have
NTLM default to OFF when HTTPS is disabled to make life easier
and to fix our OSSFuzz builds failing.

attr: rename constants and macros for consistency

Our enumeration values are not generally suffixed with `T`.  Further,
our enumeration names are generally more descriptive.

Change API instances of `fromnoun` to `from_noun` (with an underscore)

object: rename git_object__size to git_object_size

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the tag function for
consistency with the rest of the library.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the index functions for
consistency with the rest of the library.

We don't use double-underscores in the public API.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the blob functions for
consistency with the rest of the library.

The only function that is named `issomething` (without underscore) was
`git_oid_iszero`.  Rename it to `git_oid_is_zero` for consistency with
the rest of the library.

Replace fnmatch with wildmatch

Documentation fixes

Docurium seems to choke on this header because it can't see both
git_indexer_progress & git_indexer_progress_cb, let's add the include.

The `fnmatch` code has now been completely replaced by
`wildmatch`, same as upstream git.git has been doing in 2014.
Remove it.

Upstream git has converted to use `wildmatch` instead of
`fnmatch`. Convert our gitattributes logic to use `wildmatch` as
the last user of `fnmatch`. Please, don't expect I know what I'm
doing here: the fnmatch parser is one of the most fun things to
play around with as it has a sh*tload of weird cases. In all
honesty, I'm simply relying on our tests that are by now rather
comprehensive in that area.

The conversion actually fixes compatibility with how git.git
parser "**" patterns when the given path does not contain any
directory separators. Previously, a pattern "**.foo" erroneously
wouldn't match a file "x.foo", while git.git would match.

Remove the new-unused LEADINGDIR/NOLEADINGDIR flags for
`git_attr_fnmatch`.

We currently use `p_fnmatch` to compute whether a given "gitdir:"
or "gitdir/i:" conditional matches the current configuration file
path. As git.git has moved to use `wildmatch` instead of
`p_fnmatch` throughout its complete codebase, we evaluate
conditionals inconsistently with git.git in some special cases.

Convert `p_fnmatch` to use `wildmatch`. The `FNM_LEADINGDIR` flag
cannot be translated to `wildmatch`, but in fact git.git doesn't
use it here either. And in fact, dropping it while we go
increases compatibility with git.git.

When evaluating "gitdir:" and "gitdir/i:" conditionals, we
currently compare the given pattern with the value of
`git_repository_path`. Thing is though that `git_repository_path`
returns the gitdir path with trailing '/', while we actually need
to match against the gitdir without it.

Fix this issue by stripping the trailing '/' previous to
matching. Add various tests to ensure we get this right.

The function `do_match_gitdir` has some horribly named parameters
and variables. Rename them to improve readability. Furthermore,
fix a potentially undetected out-of-memory condition when
appending "**" to the pattern.

Upstream git.git has converted its codebase to use wildcard in
favor of fnmatch in commit 70a8fc999d (stop using fnmatch (either
native or compat), 2014-02-15). To keep our own regex-matching in
line with what git does, convert all trivial instances of
`fnmatch` usage to use `wildcard`, instead. Trivial usage is
defined to be use of `fnmatch` with either no flags or flags that
have a 1:1 equivalent in wildmatch (PATHNAME, IGNORECASE).

We're about to phase out our bundled fnmatch implementation as
git.git has moved to wildmatch long ago in 2014. To make it
easier to spot which files are stilll using fnmatch, remove the
implicit "fnmatch.h" include in "posix.h" and instead include it
explicitly.

In commit 70a8fc999d (stop using fnmatch (either native or
compat), 2014-02-15), upstream git has switched over all code
from their internal fnmatch copy to its new wildmatch code. We
haven't followed suit, and thus have developed some
incompatibilities in how we match regular expressions.

Import git's wildmatch from v2.22.0 and add a test suite based on
their t3070-wildmatch.sh tests.

Removal of `p_fallocate`

By now, we have repeatedly failed to provide a nice
cross-platform implementation of `p_fallocate`. Recent tries to
do that escalated quite fast to a set of different CMake checks,
implementations, fallbacks, etc., which started to look real
awkward to maintain. In fact, `p_fallocate` had only been
introduced in commit 4e3949b7 (tests: test that largefiles can
be read through the tree API, 2019-01-30) to support a test with
large files, but given the maintenance costs it just seems not to
be worht it.

As we have removed the sole user of `p_fallocate` in the previous
commit, let's drop it altogether.

The `p_fallocate` platform is currently in use in our tests,
only, but it proved to be quite burdensome to get it implemented
in a cross-platform way. The only "real" user is the test
object::tree::read::largefile, where it's used to allocate a
large file in the filesystem only to commit it to the repo and
read its object back again. We can simplify this quite a bit by
just using an in-memory buffer of 4GB. Sure, this cannot be used
on platforms with low resources. But creating 4GB files is not
any better, and we already skip the test if the environment
variable "GITTEST_INVASIVE_FS_SIZE" is not set. So we're arguably
not worse off than before.

Modularize our TLS & hash detection

The interactions between `USE_HTTPS` and `SHA1_BACKEND` have been
streamlined. Previously we would have accepted not quite working
configurations (like, `-DUSE_HTTPS=OFF -DSHA1_BACKEND=OpenSSL`) and, as
the OpenSSL detection only ran with `USE_HTTPS`, the link would fail.

The detection was moved to a new `USE_SHA1`, modeled after `USE_HTTPS`,
which takes the values "CollisionDetection/Backend/Generic", to better
match how the "hashing backend" is selected, the default (ON) being
"CollisionDetection".

Note that, as `SHA1_BACKEND` is still used internally, you might need to
check what customization you're using it for.