Merge branch 'jk/ref-cache-non-repository-optim' into maint

The underlying machinery used by "ls-files -o" and other commands
have been taught not to create empty submodule ref cache for a
directory that is not a submodule.  This removes a ton of wasted
CPU cycles.

* jk/ref-cache-non-repository-optim:
  resolve_gitlink_ref: ignore non-repository paths
  clean: make is_git_repository a public function

t/perf/p7300-clean.sh

@@ -28,4 +28,8 @@ test_perf 'clean many untracked sub dirs, ignore nested git' '
 	git clean -n -q -f -f -d 100000_sub_dirs/
 '
 
+test_perf 'ls-files -o' '
+	git ls-files -o
+'
+
 test_done

t/t3000-ls-files-others.sh

@@ -65,6 +65,13 @@ test_expect_success '--no-empty-directory hides empty directory' '
 	test_cmp expected3 output
 '
 
+test_expect_success 'ls-files --others handles non-submodule .git' '
+	mkdir not-a-submodule &&
+	echo foo >not-a-submodule/.git &&
+	git ls-files -o >output &&
+	test_cmp expected1 output
+'
+
 test_expect_success SYMLINKS 'ls-files --others with symlinked submodule' '
 	git init super &&
 	git init sub &&