odb: check for write failures

SHA256: early preparation

revparse: support bare '@'

C90: add inline macro to xdiff and mbedtls

merge: comment conflicts lines in MERGE_MSG

merge: support zdiff3 conflict styles

ci: improve the name in CI runs

git has started adding comment markers to its conflict lines in
MERGE_MSG.  Match that behavior.

A bare '@' revision syntax represents HEAD.  Support it as such.

Disabling setting `CMAKE_FIND_LIBRARY_SUFFIXES` on Apple platforms.

refs: Speed up packed lookups.

Update to the xdiff used in git v2.35.0, with updates to our build
configuration to ignore the sort of warnings that we normally care
about (signed/unsigned mismatch, unused, etc.)

Any git-specific abstraction bits are now redefined for our use in
`git-xdiff.h`.  It is a (wildly optimistic) hope that we can use that
indirection layer to standardize on a shared xdiff implementation.

Deprecate the `git_packfile_hash` function.  Callers should use the new
`git_packfile_name` function which provides a unique packfile name.

The index's checksum is not an object ID, so we should not use the
`git_oid` type.  Use a byte array for checksum calculation and storage.

Deprecate the `git_indexer_hash` function.  Callers should use the new
`git_indexer_name` function which provides a unique packfile name.

The index's checksum is not an object ID, so we should not use the
`git_oid` type.  Use a byte array for checksum calculation and storage.

Deprecate the `git_index_checksum` function without a replacement.  This
is an abstraction that callers should not care about (and indeed do not
seem to be using).

Remove the unused `git_index__changed_relative_to` function.

A simple hash-to-hexadigit formatter.

The `git_oid_tostr_s` helper is indeed helpful, unless you are using
printf debugging (by inserting more `git_oid_tostr_s` calls) shortly
after using it.  Avoid it before invoking complex functions.

Underscores in filenames in tests indicate a logical separation - so
`commit_graph` becomes `commit::graph`.  Rename it to `commitgraph` to
avoid this.

COPYING: remove regex copyright, add PCRE copyright

We now bundle PCRE for our regular expression implementation; include
that copyright notice and remove the now-unused regular expression
copyright.

diff_driver: split global_drivers array into separate elements

win32: update git for windows compatibility

C99 doesn't allow structures with flexible length array to be
placed in an array.

Improve documentation

We look for a Git for Windows installation to use its git config,
so that clients built on libgit2 can interoperate with the Git for
Windows CLI (and clients that are built on top of _it_).

Look for `git` both in the `PATH` and in the registry.  Use the _first_
git install in the path, and the first git install in the registry.

Look in both the `etc` dir and the architecture-specific `etc` dirs
(`mingw64/etc` and `mingw32/etc`) beneath the installation root.

Prefer the git in the `PATH` to the git location in the registry so that
users can override that.

Include more tests for this behavior.

Provide a helper function to find an executable in the current process's
PATH.