commit: ignore additional signatures when parsing signed commits

commit: ignore additional signatures when parsing signed commits

When we create a commit with multiple signatures, neither of these
signatures includes the other.  Consequently, when we produce the
payload which has been signed so we can verify the commit, we must strip
off any other signatures, or the payload will differ from what was
signed.  Do so, and in preparation for verifying with multiple
algorithms, pass the algorithm we want to verify into
parse_signed_commit.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>