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