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