[git] Override `diff.submodule` so `git diff` output is always parseable
[git] Override `diff.submodule` so `git diff` output is always parseable

Test Plan:
Removed a submodule with `diff.submodule` set to `log`, saw
`arc diff` error; with this change, it no longer does.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T10881

Differential Revision: https://secure.phabricator.com/D17327

folsom-labs/arcanist,r4nt/arcanist,codelabs-ch/arcanist,chundongwang/arcanist,folsom-labs/arcanist,aik099/arcanist,aik099/arcanist,uber/arcanist,codelabs-ch/arcanist,phacility/arcanist,aik099/arcanist,uber/arcanist,folsom-labs/arcanist,chundongwang/arcanist,fivestars/arcanist,fivestars/arcanist,Khan/arcanist,phacility/arcanist,fivestars/arcanist,fivestars/arcanist,chundongwang/arcanist,phacility/arcanist,codelabs-ch/arcanist,r4nt/arcanist,codelabs-ch/arcanist,Khan/arcanist,codelabs-ch/arcanist,r4nt/arcanist,folsom-labs/arcanist,phacility/arcanist,r4nt/arcanist,uber/arcanist,aik099/arcanist,chundongwang/arcanist,Khan/arcanist,phacility/arcanist,Khan/arcanist,uber/arcanist