While the `DIFF_FROM_ITERATORS` does make it shorter to implement the
various `git_diff_foo_to_bar` functions, it is a complex and unreadable
beast that implicitly assumes certain local variable names. This is not
something desirable to have at all and obstructs understanding and more
importantly debugging the code by quite a bit.

The `DIFF_FROM_ITERATORS` macro basically removed the burden of having
to derive the options for both iterators from a pair of iterator flags
and the diff options. This patch introduces a new function that does the
that exact and refactors all callers to manage the iterators by
themselves.

As we potentially need to allocate a shared prefix for the
iterator, we need to tell the caller to allocate that prefix as soon as
the options aren't required anymore. Thus, the function has a `char
**prefix` out pointer that will get set to the allocated string and
subsequently be free'd by the caller.

While this patch increases the line count, I personally deem this to an
acceptable tradeoff for increased readbiblity.

clone: don't decode URL percent encodings

Security updates from 0.28.3

The function `commit_quick_parse` provides a way to quickly parse
parts of a commit without storing or verifying most of its
metadata. The first thing it does is calculating the number of
parents by skipping "parent " lines until it finds the first
non-parent line. Afterwards, this parent count is passed to
`alloc_parents`, which will allocate an array to store all the
parent.

To calculate the amount of storage required for the parents
array, `alloc_parents` simply multiplicates the number of parents
with the respective elements's size. This already screams "buffer
overflow", and in fact this problem is getting worse by the
result being cast to an `uint32_t`.

In fact, triggering this is possible: git-hash-object(1) will
happily write a commit with multiple millions of parents for you.
I've stopped at 67,108,864 parents as git-hash-object(1)
unfortunately soaks up the complete object without streaming
anything to disk and thus will cause an OOM situation at a later
point. The point here is: this commit was about 4.1GB of size but
compressed down to 24MB and thus easy to distribute.

The above doesn't yet trigger the buffer overflow, thus. As the
array's elements are all pointers which are 8 bytes on 64 bit, we
need a total of 536,870,912 parents to trigger the overflow to
`0`. The effect is that we're now underallocating the array
and do an out-of-bound writes. As the buffer is kindly provided
by the adversary, this may easily result in code execution.

Extrapolating from the test file with 67m commits to the one with
536m commits results in a factor of 8. Thus the uncompressed
contents would be about 32GB in size and the compressed ones
192MB. While still easily distributable via the network, only
servers will have that amount of RAM and not cause an
out-of-memory condition previous to triggering the overflow. This
at least makes this attack not an easy vector for client-side use
of libgit2.

When the VirtualStore feature is in effect, it is safe to let random
users write into C:\ProgramData because other users won't see those
files. This seemed to be the case when we introduced support for
C:\ProgramData\Git\config.

However, when that feature is not in effect (which seems to be the case
in newer Windows 10 versions), we'd rather not use those files unless
they come from a trusted source, such as an administrator.

This change imitates the strategy chosen by PowerShell's native OpenSSH
port to Windows regarding host key files: if a system file is owned
neither by an administrator, a system account, or the current user, it
is ignored.

Will add later when infrastructure is configured

stash: avoid recomputing tree when committing worktree

Variadic macros

Add sign capability to git_rebase_commit

remote: remove unused block of code

Adjust printf specifiers in examples code

In "remote.c", we have a chunk of code that is #ifdef'fed out via
`#if 0` with a comment that we could export it as a helper function.
The code was implemented in 2013 and ifdef'fed in 2014, which shows that
there's clearly no interest in having such a helper at all.

As this block has recently created some confusion about `p_getenv` due
to it containing the only reference to that function in our codebase,
let's remove this block altogether.

config: check if we are running in a sandboxed environment

Static analysis of example code found multiple findings of `printf` usage
where filling value is members of git_indexer_progress object. Specifier
used was for signed int but git_indexer_progress members are typed as
unsigned ints. `printf` specifiers were altered to match type.

On macOS the $HOME environment variable returns the path to the sandbox container instead of the actual user $HOME for sandboxed apps. To get the correct path, we have to get it from the password file entry.

Fix example checkout to forbid rather than require --

editorconfig: update to match our coding style

Compare buffers in diff example

The macro `p_snprintf` is implemented as a variadic macro that
calls `snprintf` directly with `__VA_ARGS__`. In C89, variadic
macros are not allowed, but as the arguments of `p_snprintf` and
`snprintf` are matching 1:1, we can fix this by simply removing
the parameter list from `p_snprintf`.

The macro `apply_err` is implemented as a variadic macro, which
are not defined by C89. Convert it to a variadic function,
instead.

The macro `git_parse_error` is implemented in a variadic way so
that it's possible to pass printf-style parameters.
Unfortunately, variadic macros are not defined by C89 and thus we
cannot use that functionality. But as we have implemented
`git_error_vset` in the previous commit, we can now just use that
instead.

Convert `git_parse_error` to a variadic function and use
`git_error_vset` to fix the compliance violation. While at it,
move the function to "patch_parse.c".

Right now, we only provide a `git_error_set` that has a variadic
function signature. It's impossible to drive this function in a
C89-compliant way from other functions that have a variadic
signature, though, like for example `git_parse_error`.

Implement a new `git_error_vset` function that gets a `va_list`
as parameter, fixing the above problem.

Include ahead_behind in the test suite

config: separate file and snapshot backends

object: deprecate git_object__size for removal

In #5118 we remove the double-underscore to make it a normally-named public
function. However, this is not an interesting function outside of the library
and it takes up a name for something that could be more useful.

Remove the single-underscore version as we have not done any releases with it.

The internal backend structures are kind-of legacy and do not really
speak for themselves. Rename them accordingly to make them easier to
understand.

To further distinguish the file writeable and readonly backends,
separate the readonly backend into its own "config_snapshot.c"
implementation. The snapshot backend can be generically used to snapshot
any type of backend.

In `backend_readonly_free`, the passed in config backend is being cast
to a `diskfile_backend` instead of to a `diskfile_readonly_backend`.
While this works out just fine because we only access its header values,
which were shared between both backends, it is undefined behaviour.

Use the correct type to fix this.

The `diskfile_header` structure is shared between both
`diskfile_backend` and `diskfile_readonly_backend`. The separation and
resulting casting is confusing at times and a source for programming
errors.

Remove the shared structure and inline them directly.

While most functions of the readonly configuration backend are
implemented separately from the writeable configuration backend, the two
functions `config_iterator_new` and `config_get` are shared between
both. This sharing makes it necessary to have some shared data
structures, which is the `diskfile_header` structure. Unfortunately, this
makes the backends harder to grasp than necessary due to all the casting
between structs and also quite error prone.

Reimplement those functions for the readonly backends. As readonly
backends cannot be refreshed anyway, we can remove the calls to
`config_refresh` in there.

The `config_readonly_open` function currently receives as input a
diskfile backend and will copy its entries to a new snapshot. This is
rather intimate, as we need to assume that the source config backend is
in fact a diskfile entry. We can do better than this though by using
generic methods to copy contents of the provided backend, e.g. by using
a config iterator. This also allows us to decouple the read-only backend
from the read-write backend.

When duplicating a configuration entry, we allocate a new entry but do
not verify that we get a valid pointer back. As we're dereferencing the
pointer afterwards, we might thus run into a segfault in out-of-memory
situations.

Extract a new function `git_config_entries_dup_entry` that handles the
complete entry duplication. Fix the error by using
`GIT_ERROR_CHECK_ALLOC`.

Make the example program for checkout follow git syntax, where
"--" indicates a file.  This was likely just a strcmp return
value confusion.