Fix arc lint amend for git to not add untracked files
Fix arc lint amend for git to not add untracked files

Summary:
Arc lint for git is currently adding all untracked files when it
amends the commit with lint fixes. This changes the git add -A to be
git add -u. This only adds files that were already tracked. -A was adding
untracked files as well which was not the desired behavior here.

Test Plan:
Create an untracked file.
Commit a lint failure another file.

arc diff and choose to amend the lint patches.

Verify that the untracked file was not added but the tracked file was amended.

Reviewers: epriestley, wez, chad

Reviewed By: chad

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5731

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