Expose `--coverage` and `--no-coverage` flags from `arc diff`
Expose `--coverage` and `--no-coverage` flags from `arc diff`

Summary: Currently, we don't expose these at top level, so you can't disable coverage if your coverage is explosively broken. Expose them as passthrough arguments.

Test Plan:

  - Touched a file in `arc` which triggered unit tests.
  - Without `xdebug` installed:
    - Ran `arc diff --preview`, `arc diff --preview --no-coverage` (both fine).
    - Ran `arc diff --preview --coverage`, got exception about coverage not being available.
  - Installed `xdebug`.
    - Ran `arc diff --preview`, got coverage.
    - Ran `arc diff --preview --coverage`, got coverage.
    - Ran `arc diff --preview --no-coverage`, no coverage.

Reviewers: indiefan, btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4745

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