Allow `arc diff` and similar to run on systems with no `git`

Summary:
We assume `git` is available now, but should not. Specifically, if a user runs a working copy operation like `arc list` in an SVN working copy without `git` available, they get this error: P707

We interpret git errors very narrowly; be more liberal in how we interpret them. This assumes users working with `git` will have a functional `git`, but this seems like a reasonable assumption and lets us remove some error text matching code.

Test Plan: Changed `git` to `girt`, ran `arc list`, saw a reasonable exception. Changed back to `git`, saw git detected.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: svemir, aran

Differential Revision: https://secure.phabricator.com/D4804