commit: allow parse_commit* to handle any repo

commit: allow parse_commit* to handle any repo

Just like the previous commit, parse_commit and friends are used a lot
and are found in new patches, so we cannot change their signature easily.

Re-introduce these function prefixed with 'repo_' that take a repository
argument and keep the original as a shallow macro.

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>