commit: rephrase the error when user did not touch templated log message

commit: rephrase the error when user did not touch templated log message

When the user exited editor without editing the commit log template given
by "git commit -t <template>", the commit was aborted (correct) with an
error message that said "due to empty commit message" (incorrect).

This was because the original template support was done by piggybacking on
the check to detect an empty log message.  Split the codepaths into two
independent checks to clarify the error.

Signed-off-by: Junio C Hamano <gitster@pobox.com>