merge: Honor prepare-commit-msg return code

merge: Honor prepare-commit-msg return code

65969d4 (merge: honor prepare-commit-msg hook, 2011-02-14) tried to
make "git commit" and "git merge" consistent, because a merge that
required user assistance has to be concluded with "git commit", but
back then only "git commit" triggered prepare-commit-msg hook.

When it added a call to run the prepare-commit-msg hook, however, it
forgot to check the exit code from the hook like "git commit" does,
and ended up replacing one inconsistency with another.

When prepare-commit-msg hook that is run from "git merge" exits with
a non-zero status, abort the commit.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>