The git included with xenial is ancient, and lacks sha256 support.

The longest path within a git repository is now a SHA256 format
packfile. Adjust our max length checking to match.

This is much of the plumbing for the object database to support SHA256,
and for objects to be able to parse SHA256 versions of themselves.

Teach the packfile machinery to cope with SHA256.

In clar, an underscore is meaningful; avoid using it incorrectly /
unnecessarily.

This is a conversion of the testrepo.git to SHA256 support.

Ensure that we maintain the `core.repositoryFormatVersion` value instead
of always overwriting it with the default.

Provide an internal function to set the repository's `objectformat`,
both in the internal object and in the configuration.

Teach the repository about the `objectformat` extension, supporting
`sha1` always and `sha256` when the experimental sha256 support is
active.

Support OpenSSL3: add OpenSSL deprection warning compatibility flag

Add `OPENSSL_API_COMPAT=0x10100000L` since we use the now-deprecated
low-level hash functions.

ci: always create test summaries, even on failure

When the dependent jobs fail -- possibly due to test failures -- we
should still produce the job summary that shows those test failures.

tests: update clar test runner

Abstract time counter for tests; use gettimeofday on Unix and
GetTickCount on Windows.

Update to the latest main version of clar, which includes improved xml
summary output.

src: hide unused hmac() prototype

The builtin hash uses the code verbatim from rfc6234, including
prototypes for functions that we don't use (like hmac). Remove all
unused prototypes to avoid collisions with things that an operating
system might provide (like hmac).

It conflicts with NetBSD's in its libc.

Closes #6457

push: use resolved oid as the source

211c9719 attempts to use the parsed OID
but inverted the arguments to `git_oid_cpy`.

push: revparse refspec source, so you can push things that are not refs

thread: avoid warnings when building without threads