tests: sanitize more git environment variables

These variables should generally not be set in one's
environment, but they do get set by rebase, which means
doing an interactive rebase like:

  pick abcd1234 foo
  exec make test

will cause false negatives in the test suite.

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

t/test-lib.sh

@@ -70,6 +70,9 @@ unset GIT_NOTES_REF
 unset GIT_NOTES_DISPLAY_REF
 unset GIT_NOTES_REWRITE_REF
 unset GIT_NOTES_REWRITE_MODE
+unset GIT_REFLOG_ACTION
+unset GIT_CHERRY_PICK_HELP
+unset GIT_QUIET
 GIT_MERGE_VERBOSITY=5
 export GIT_MERGE_VERBOSITY
 export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME