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

Fix a minor bug introduced in bc40ce4de6 (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 6098817fd7 (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>

builtin/merge.c

@@ -845,15 +845,18 @@ static void prepare_to_commit(struct commit_list *remoteheads)
 	struct strbuf msg = STRBUF_INIT;
 	const char *index_file = get_index_file();
 
-	if (!no_verify && run_commit_hook(0 < option_edit, index_file, "pre-merge-commit", NULL))
-		abort_commit(remoteheads, NULL);
-	/*
-	 * Re-read the index as pre-merge-commit hook could have updated it,
-	 * and write it out as a tree.  We must do this before we invoke
-	 * the editor and after we invoke run_status above.
-	 */
-	if (hook_exists("pre-merge-commit"))
-		discard_cache();
+	if (!no_verify) {
+		if (run_commit_hook(0 < option_edit, index_file,
+				    "pre-merge-commit", NULL))
+			abort_commit(remoteheads, NULL);
+		/*
+		 * Re-read the index as pre-merge-commit hook could have updated it,
+		 * and write it out as a tree.  We must do this before we invoke
+		 * the editor and after we invoke run_status above.
+		 */
+		if (hook_exists("pre-merge-commit"))
+			discard_cache();
+	}
 	read_cache_from(index_file);
 	strbuf_addbuf(&msg, &merge_msg);
 	if (squash)