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

folsom-labs/arcanist,Khan/arcanist,haskell-infra/arcanist,fivestars/arcanist,fivestars/arcanist,sneakpeeq/arcanist,chundongwang/arcanist,alexmv/arcanist,wikimedia/phabricator-arcanist,alexmv/arcanist,coursera/arcanist,dguo-coursera/arcanist,coursera/arcanist,fivestars/arcanist,r4nt/arcanist,kevinseelbach/arcanist,Symplicity/arcanist,eSpark/arcanist,phacility/arcanist,zhihu/arcanist,wxstars/arcanist,codelabs-ch/arcanist,wikimedia/phabricator-arcanist,sneakpeeq/arcanist,uber/arcanist,wxstars/arcanist,aik099/arcanist,sneakpeeq/arcanist,memsql/arcanist,jra3/arcanist,aik099/arcanist,fivestars/arcanist,kevinseelbach/arcanist,hach-que/arcanist,wharris/arcanist,kevinseelbach/arcanist,memsql/arcanist,codelabs-ch/arcanist,julkiewicz/arcanist,jra3/arcanist,zhihu/arcanist,aik099/arcanist,kevinseelbach/arcanist,Khan/arcanist,KieranHayes/arcanist,folsom-labs/arcanist,calblueprint/arcanist,Symplicity/arcanist,zhihu/arcanist,r4nt/arcanist,haskell-infra/arcanist,julkiewicz/arcanist,uber/arcanist,PageUpPeopleOrg/arcanist-patches,jra3/arcanist,KieranHayes/arcanist,hach-que/arcanist,wharris/arcanist,wikimedia/phabricator-arcanist,wxstars/arcanist,wharris/arcanist,KieranHayes/arcanist,chundongwang/arcanist,alexmv/arcanist,haskell-infra/arcanist,PageUpPeopleOrg/arcanist-patches,haskell-infra/arcanist,phacility/arcanist,Khan/arcanist,folsom-labs/arcanist,julkiewicz/arcanist,eSpark/arcanist,phacility/arcanist,memsql/arcanist,eSpark/arcanist,aik099/arcanist,PageUpPeopleOrg/arcanist-patches,calblueprint/arcanist,chundongwang/arcanist,sneakpeeq/arcanist,wxstars/arcanist,julkiewicz/arcanist,coursera/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,hach-que/arcanist,zhihu/arcanist,eSpark/arcanist,alexmv/arcanist,chundongwang/arcanist,uber/arcanist,folsom-labs/arcanist,wharris/arcanist,calblueprint/arcanist,hach-que/arcanist,wikimedia/phabricator-arcanist,phacility/arcanist,Khan/arcanist,phacility/arcanist,memsql/arcanist,Symplicity/arcanist,jra3/arcanist,dguo-coursera/arcanist,dguo-coursera/arcanist,r4nt/arcanist,PageUpPeopleOrg/arcanist-patches,Symplicity/arcanist,dguo-coursera/arcanist,uber/arcanist,r4nt/arcanist,coursera/arcanist,codelabs-ch/arcanist,calblueprint/arcanist,KieranHayes/arcanist