commit: convert mergetag before computing the signature of a commit

commit: convert mergetag before computing the signature of a commit

It so happens that commit mergetag lines embed a tag object.  So to
compute the compatible signature of a commit object that has mergetag
lines the compatible embedded tag must be computed first.

Implement this by duplicating and converting the commit extra headers
into the compatible version of the commit extra headers, that need
to be passed to commit_tree_extended.

To handle merge tags only the compatible extra headers need to be
computed.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>