Detect LFS by looking for tracks in ".gitattributes" instead of using "ls-tree"

Summary:
See PHI1718. See also https://discourse.phabricator-community.org/t/arc-diff-fails-due-to-git-cmd-fails/3680/.

Currently, arc diff detects Git LFS with git ls-files -z -- ':(attr:filter=lfs)' magic. This is an accurate test, but does not work on older Git.

Try a simpler, dumber test and see if that will work. If this also has issues, we can try this stuff:

• do version detection;
• pipe the whole tree to git check-attr;
• try a command like git lfs ls-files instead, which is probably a wrapper on one of these other commands.

Test Plan:

• In a non-LFS repository, ran "arc diff" and saw the repository detect as non-LFS.
• In an LFS repository, ran "arc diff" and saw the repository detect as LFS.

Differential Revision: https://secure.phabricator.com/D21190