object-file-convert: convert commit objects when writing

object-file-convert: convert commit objects when writing

When writing a commit object in a repository with both SHA-1 and
SHA-256, we'll need to convert our commit objects so that we can write
the hash values for both into the repository.  To do so, let's add a
function to convert commit objects.

Read the commit object and map the tree value and any of the parent
values, and copy the rest of the commit through unmodified.  Note that
we don't need to modify the signature headers, because they are the
same under both algorithms.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>