.gitignore: test-hashmap is a generated file

Prevent the "test-hashmap" program from being accidentally tracked
with "git add" or cluttering "git status" output.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.gitignore

@@ -185,6 +185,7 @@
 /test-dump-cache-tree
 /test-scrap-cache-tree
 /test-genrandom
+/test-hashmap
 /test-index-version
 /test-line-buffer
 /test-match-trees