ArcanistGitAPI: remove a bogus comment

See: <https://github.com/facebook/arcanist/commit/628de7d7a15cd7b5737515ec2831b3b4c3f601a5#commitcomment-4675543>

This comment is indecipherable even if it originally meant something.

Reviewed by: epriestley
ArcanistGitAPI: document a git ls-files bug

When a submodule is ignored (ignore=all in .gitconfig),

  $ git ls-files -m

fails to exclude the submodule from the listing. Other commands like

  $ git diff-index --name-only HEAD

exclude it just fine.

See: <https://github.com/facebook/arcanist/pull/120>

Reviewed by: epriestley