Detect repo for 'browse'; don't require project ID

Test Plan: Ran 'arc browse' in a repo that contains a .arcconfig and one without.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8173
Support browsing '.' and default to it

Test Plan: Ran 'arc browse', 'arc browse .', 'arc browse main.py'.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, avivey

Differential Revision: https://secure.phabricator.com/D8174
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
Merge branch 'master' of http://github.com/facebook/arcanist