Restore the ":(attr:filter=lfs)" test for LFS

Summary:
See D21190. The ".gitattributes" approach fails when ".gitattributes" is in a subdirectory (or global). These are probably unusual cases, but at least one is known in the wild.

Instead:

• Restore the ":(attr:filter=lfs)" test, which seems to be the fastest accurate test available in modern Git.
• If the test fails, assume the repository is not LFS. This only impacts users running very old versions of Git.

Test Plan:

• In LFS and non-LFS repositories, created diffs. Saw correct detection again.
• Broke the command on purpose, saw LFS detection conclude "no LFS", but not fail disastrously.

Subscribers: ptarjan

Differential Revision: https://secure.phabricator.com/D21192