tests: remove leftover untracked files

Remove untracked files that are unwanted after they are done being used.

While the set of cases in this patch is certainly far from
comprehensive, it was motivated by some work to see what the fallout
would be if we were to make the removal of untracked files as a side
effect of other commands into an error.  Some cases were a bit more
involved than the testcase changes included in this patch, but the ones
included here represent the simple cases.  While this patch is not that
important since we are not changing the behavior of those other commands
into an error in the near term, I thought these changes were useful
anyway as an explicit documentation of the intent that these untracked
files are no longer useful.

Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Acked-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6424-merge-unrelated-index-changes.sh

@@ -132,6 +132,7 @@ test_expect_success 'merge-recursive, when index==head but head!=HEAD' '
 
 	# Make index match B
 	git diff C B -- | git apply --cached &&
+	test_when_finished "git clean -fd" &&  # Do not leave untracked around
 	# Merge B & F, with B as "head"
 	git merge-recursive A -- B F > out &&
 	test_i18ngrep "Already up to date" out