Explicitly use "--format=medium" when running 'git log'
Explicitly use "--format=medium" when running 'git log'

Summary:
Git (the world's hardest revision control system) allows you to change
output formats by accident and/or without your direct knowledge. Protect users
from themselves.

Test Plan:
Changed "pretty" in [format] to "format:quack" so every log just
outputs the word "quack". Ran "arc diff" successfully.

Reviewed By: aran
Reviewers: aran
CC: epriestley, aran
Differential Revision: 56

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