merge: don't run post-hook logic on --no-verify

merge: don't run post-hook logic on --no-verify

Fix a minor bug introduced in bc40ce4de61 (merge: --no-verify to
bypass pre-merge-commit hook, 2019-08-07), when that change made the
--no-verify option bypass the "pre-merge-commit" hook it didn't update
the corresponding find_hook() (later hook_exists()) condition.

As can be seen in the preceding commit in 6098817fd7f (git-merge:
honor pre-merge-commit hook, 2019-08-07) the two should go hand in
hand. There's no point in invoking discard_cache() here if the hook
couldn't have possibly updated the index.

It's buggy that we use "hook_exist()" here, and as discussed in the
subsequent commit it's subject to obscure race conditions that we're
about to fix, but for now this change is a strict improvement that
retains any caveats to do with the use of "hooks_exist()" as-is.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>