push: Prepare pack before sending pack header.

odb: check for write failures

Co-authored-by: Edward Thomson <ethomson@github.com>

For large pushes, preparing the pack can take a while. Currently we
send the pack header first, followed by preparing the pack and then
finally sending the pack. Unfortunately github.com will terminate
a git-receive-pack command over http if it is idle for more than 10
seconds. This is easily exceeded for a large push, and so the push is
rejected with a Broken Pipe error.

This patch moves the pack preparation ahead of sending the pack header,
so that the timeout is avoided.

prepare_pack() can be called multiple times but will only do the work
once, so the original PREPARE_PACK call inside git_packbuilder_foreach()
remains.

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