commit: sign commit after mutating buffer

commit: sign commit after mutating buffer

The ensure_utf8 function can mutate the buffer to change its encoding,
so we must call it before signing the buffer so that we do not
invalidate the signature, which is made over raw bytes.  Fix a bug which
caused the compatibility code to not convert the compatibility buffer if
the main buffer was invalid UTF-8.  We expect both buffers to be valid
UTF-8 or both invalid, since the only data that would differ between
them would be hex object IDs, which are always valid UTF-8.

Add a test for this case using 0xfe and 0xff, which are never valid in
UTF-8.

Reported-by: Kushal Das <kushal@sunet.se>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>