Win32 path canonicalization refactoring

Check object existence when creating a tree from an index

The testrepo test fixture has an index file that's damaged, missing an
object.  The index previously had an entry of `src/index.c` with id
3161df8cbf3a006b4ef85be6497a0ea6bde98541, but that object was missing in
the repository.  This commit adds an object to the repository and
updates the index to use that existing blob.

Similarly, the index has an entry for `readme` with an id of
97328ac7e3bd0bcd3900cb3e7a624d71dd0df888.  This can be restored from
other test repositories.

With these fixed, now the write tree from index tests can pass since they
validate object existence.

Ninja build

docs: fix transparent/opaque confusion in the conventions file

Configuration variables can appear on the same line as the section header

Update `git_win32_path_remove_namespace` to disambiguate the prefix
being removed versus the prefix being added.  Now we remove the
"namespace", and (may) add a "prefix" in its place.  Eg, we remove the
`\\?\` namespace.  We remove the `\\?\UNC\` namespace, and replace it
with the `\\` prefix.  This aids readability somewhat.

Additionally, use pointer arithmetic instead of offsets, which seems to
also help readability.

The internal API `git_win32__canonicalize_path` is far, far too easily
confused with the internal API `git_win32_path_canonicalize`.  The
former removes the namespace prefix from a path (eg, given
`\\?\C:\Temp\foo`, it returns `C:\Temp\foo`, and given
`\\?\UNC\server\share`, it returns `\\server\share`).  As such, rename
it to `git_win32_path_remove_namespace`.

`git_win32_path_canonicalize` remains unchanged.

path: export the dotgit-checking functions

cmake: correct comment from libssh to libssh2

We use libssh2.  We do not use libssh.  Make sure to disambiguate them
correctly.

Object parsing fuzzer

These checks are preformed by libgit2 on checkout, but they're also useful for
performing checks in applications which do not involve checkout.

Expose them under `sys/` as it's still fairly in the weeds even for this
library.

While rare and a machine would typically not generate such a configuration file,
it is nevertheless valid to write

    [foo "bar"] baz = true

and we need to deal with that instead of assuming everything is on its own line.

config: Port config_file_fuzzer to the new in-memory backend.

Add a simple fuzzer that exercises our object parser code. The fuzzer
is quite trivial in that it simply passes the input data directly to
`git_object__from_raw` for each of the four object types.

When failing to parse a raw object fromits data, we free the
partially parsed object but then fail to propagate the error to the
caller. This may lead callers to operate on objects with invalid memory,
which will sooner or later cause the program to segfault.

Fix the issue by passing up the error code returned by `parse_raw`.

The standalone driver for libgit2's fuzzing targets makes use of
functions from libgit2 itself. While this is totally fine to do, we need
to make sure to always have libgit2 initialized via `git_libgit2_init`
before we call out to any of these. While this happens in most cases as
we call `LLVMFuzzerInitialize`, which is provided by our fuzzers and
which right now always calls `git_libgit2_init`, one exception to this
rule is our error path when not enough arguments have been given. In
this case, we will call `git_vector_free_deep` without libgit2 having
been initialized. As we did not set up our allocation functions in that
case, this will lead to a segmentation fault.

Fix the issue by always initializing and shutting down libgit2 in the
standalone driver. Note that we cannot let this replace the
initialization in `LLVMFuzzerInitialize`, as it is required when using
the "real" fuzzers by LLVM without our standalone driver. It's no
problem to call the initialization and deinitialization functions
multiple times, though.

We have two similar functions, `git_treebuilder_insert` and `append_entry` which
are used in different codepaths as part of creating a new tree. The former
learnt to check for object existence under strict object creation, but the
latter did not.

This allowed the creation of a tree from an unowned index to bypass some of the
checks and create a tree pointing to a nonexistent object.

Extract a single function which performs these checks and call it from both
codepaths. In `append_entry` we still do not validate when asked not to, as this
is data which is already in the tree and we want to allow users to deal with
repositories which already have some invalid data.

When the index does not belong to any repository, we do not do any checks of the
target id going in as we cannot verify that it exists.

When we then write it out to a repository as a tree, we fail to perform the
object existance and type-matching check that we do in other code-paths. This
leads to being able to write trees which point to non-existent blobs even with
strict object creation enabled.

Add some more tests for git_futils_rmdir_r and some cleanup

diff_stats: use git's formatting of renames with common directories

ignore unsupported http authentication contexts

auth_context_match returns 0 instead of -1 for unknown schemes to
not fail in situations where some authentication schemes are supported
and others are not.

apply_credentials is adjusted to handle auth_context_match returning
0 without producing authentication context.

submodule: ignore path and url attributes if they look like options