rev-list: disable --use-bitmap-index when pruning commits

The reachability bitmaps do not have enough information to
tell us which commits might have changed path "foo", so the
current code produces wrong answers for:

  git rev-list --use-bitmap-index --count HEAD -- foo

(it silently ignores the "foo" limiter). Instead, we should
fall back to doing a normal traversal (it is OK to fall
back rather than complain, because --use-bitmap-index is a
pure optimization, and might not kick in for other reasons,
such as there being no bitmaps in the repository).

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

t/t5310-pack-bitmaps.sh

@@ -53,6 +53,12 @@ rev_list_tests() {
 		test_cmp expect actual
 	'
 
+	test_expect_success "counting commits with limiting ($state)" '
+		git rev-list --count HEAD -- 1.t >expect &&
+		git rev-list --use-bitmap-index --count HEAD -- 1.t >actual &&
+		test_cmp expect actual
+	'
+
 	test_expect_success "enumerate --objects ($state)" '
 		git rev-list --objects --use-bitmap-index HEAD >tmp &&
 		cut -d" " -f1 <tmp >tmp2 &&