Allow `arc unit --everything` to work without a base commit
Allow `arc unit --everything` to work without a base commit

Summary: Fixes T2461. Similarly to `arc lint --everything`, `arc unit --everything` should work without a base commit.

Test Plan: Removed the `.git/arc/default-relative-commit` file and ran `arc unit --everything`.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T2461

Differential Revision: https://secure.phabricator.com/D11459

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