t3419: prevent failure when run with EXPENSIVE

This test runs a function which itself runs several assertions.  The
last of these assertions cleans up the .git/rebase-apply directory,
since when run with EXPENSIVE set, the function is invoked a second time
to run the same tests with a larger data set.

However, as of 2ac0d6273f ("rebase: change the default backend from "am"
to "merge"", 2020-02-15), the default backend of rebase has changed, and
cleaning up the rebase-apply directory has no effect: it no longer
exists, since we're using rebase-merge instead.

Since we don't really care which rebase backend is in use, let's just
use the command "git rebase --quit", which will do the right thing
regardless.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3419-rebase-patch-id.sh

@@ -90,7 +90,7 @@ do_tests () {
 		git commit -q -m squashed &&
 		git checkout -q other^{} &&
 		test_must_fail git rebase squashed &&
-		rm -rf .git/rebase-apply
+		git rebase --quit
 	'
 }