Skip new_file_size non-zero test, custom error message if file changed in workdir

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

"diff_file_content_load_workdir_file()" maps a file from the workdir
into memory. It uses git_diff_file.size to determine the size of the
memory mapping.

If this value goes stale, the mmaped area would be sized incorrectly.
This could occur if an external program changes the contents of the
file after libgit2 had cached its size. This used to segfault if the
file becomes smaller (mmaped area too large).

This patch causes diff_file_content_load_workdir_file to fail without
crashing if it detects that the file size has changed.

Fetch by object id

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

Provide a mechanism for (internal) users to determine a remote's
capabilities from the transport.

Move the functionality to update an individual tip out of the loop;
although the update tip function remains rather gnarly, at least the
outer function is a bit less onerous.

Pull ref updating into its own function for future uses.

Introduce a function that determines whether a given string is a valid
object id (40 chars of hexadigits).

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.