resolve_gitlink_ref: ignore non-repository paths

When we want to look up a submodule ref, we use
get_ref_cache(path) to find or auto-create its ref cache.
But if we feed a path that isn't actually a git repository,
we blindly create the ref cache, and then may die deeper in
the code when we try to access it. This is a problem because
many callers speculatively feed us a path that looks vaguely
like a repository, and expect us to tell them when it is
not.

This patch teaches resolve_gitlink_ref to reject
non-repository paths without creating a ref_cache. This
avoids the die(), and also performs better if you have a
large number of these faux-submodule directories (because
the ref_cache lookup is linear, under the assumption that
there won't be a large number of submodules).

To accomplish this, we also break get_ref_cache into two
pieces: the lookup and auto-creation (the latter is lumped
into create_ref_cache). This lets us first cheaply ask our
cache "is it a submodule we know about?" If so, we can avoid
repeating our filesystem lookup. So lookups of real
submodules are not penalized; they examine the submodule's
.git directory only once.

The test in t3000 demonstrates a case where this improves
correctness (we used to just die). The new perf case in
p7300 shows off the speed improvement in an admittedly
pathological repository:

Test                  HEAD^               HEAD
----------------------------------------------------------------
7300.4: ls-files -o   66.97(66.15+0.87)   0.33(0.08+0.24) -99.5%

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

1 files changed, 7 insertions, 0 deletions

diff --git a/t/t3000-ls-files-others.sh b/t/t3000-ls-files-others.sh
index 88be904c09..c525656b2c 100755
--- a/t/t3000-ls-files-others.sh
+++ b/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 &&