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>