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