t4200: give us a clean slate after "rerere gc" tests

The "multiple identical conflicts" test counts the number of entries
in the rerere database after trying a handful of mergy operations
and recording their resolutions, but without initializing the rerere
database to a known state, allowing the state left by previous tests
to trigger a false failure.  Make it robust by cleaning the database
before it starts.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4200-rerere.sh

@@ -446,6 +446,8 @@ merge_conflict_resolve () {
 }
 
 test_expect_success 'multiple identical conflicts' '
+	rm -fr .git/rr-cache &&
+	mkdir .git/rr-cache &&
 	git reset --hard &&
 
 	test_seq 1 6 >early &&