Minor modernizations to `arc browse`
Minor modernizations to `arc browse`

Summary:
Do a little cleanup:

  - Remove copyright header (we removed all of these a long time ago, this one just snuck through somehow).
  - Remove `@group` comment (obsolete with new Diviner).
  - Note support for all VCSes.
  - Add pht() for translation.
  - Hint `arc browse .`.
  - Fail on no paths sooner.
  - Raise a useful error if we can't figure out which repository we're heading to.
  - Clarify "open" comment.
  - Use `Filesystem::binaryExists()`.
  - Some minor wordsmithing.

Test Plan: `arc browse`, `arc browse .`, `arc browse README`, `arc browse README src`, ran `arc browse` in valid working copy with no associated repo.

Reviewers: btrahan, spicyj

Reviewed By: spicyj

CC: aran

Differential Revision: https://secure.phabricator.com/D8176

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