commit: do not complain of empty messages from -C

commit: do not complain of empty messages from -C

When we pick another commit's message, we die() immediately
if we find that it's empty and we are not going to run an
editor (i.e., when running "-C" instead of "-c").  However,
this check is redundant and harmful.

It's redundant because we will already notice the empty
message later, after we would have run the editor, and die
there (just as we would for a regular, not "-C" case, where
the user provided an empty message in the editor).

It's harmful for a few reasons:

  1. It does not respect --allow-empty-message. As a result,
     a "git rebase -i" cannot "pick" such a commit. So you
     cannot even go back in time to fix it with a "reword"
     or "edit" instruction.

  2. It does not take into account other ways besides the
     editor to modify the message. For example, "git commit
     -C empty-commit -m foo" could take the author
     information from empty-commit, but add a message to it.
     There's more to do to make that work correctly (and
     right now we explicitly forbid "-C with -m"), but this
     removes one roadblock.

  3. The existing check is not enough to prevent segfaults.
     We try to find the "\n\n" header/body boundary in the
     commit. If it is at the end of the string (i.e., no
     body), _or_ if we cannot find it at all (i.e., a
     truncated commit object), we consider the message
     empty. With "-C", that's OK; we die in either case. But
     with "-c", we continue on, and in the case of a
     truncated commit may end up dereferencing NULL+2.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>