commit: force commit to parse from object database

commit: force commit to parse from object database

In anticipation of verifying commit-graph file contents against the
object database, create parse_commit_internal() to allow side-stepping
the commit-graph file and parse directly from the object database.

Due to the use of generation numbers, this method should not be called
unless the intention is explicit in avoiding commits from the
commit-graph file.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>