commit-reach: prepare get_merge_bases to handle any repo

commit-reach: prepare get_merge_bases to handle any repo

Similarly to previous patches, the get_merge_base functions are used
often in the code base, which makes migrating them hard.

Implement the new functions, prefixed with 'repo_' and hide the old
functions behind a wrapper macro.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>