t7700: clean up .keep file in bitmap-writing test

After our test snippet finishes, the .keep file is left in place, making
it hard to do further tests of the auto-bitmap-writing code (since it
suppresses the feature completely). Let's clean it up.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7700-repack.sh

@@ -243,6 +243,7 @@ test_expect_success 'no bitmaps created if .keep files present' '
 	pack=$(ls bare.git/objects/pack/*.pack) &&
 	test_path_is_file "$pack" &&
 	keep=${pack%.pack}.keep &&
+	test_when_finished "rm -f \"\$keep\"" &&
 	>"$keep" &&
 	git -C bare.git repack -ad &&
 	find bare.git/objects/pack/ -type f -name "*.bitmap" >actual &&