Add t/helper/test-write-cache to .gitignore

This new binary was introduced in commit 3921a0b ("perf: add test for
writing the index", 2017-08-21), but a .gitignore entry was not added
for it. Add that entry.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/.gitignore

@@ -35,3 +35,4 @@
 /test-svn-fe
 /test-urlmatch-normalization
 /test-wildmatch
+/test-write-cache