Allow "arc browse" to browse objects

Summary: Ref T5781. This implements `arc browse T234`, for any monogrammed object.

Test Plan:
  - Ran `arc browse T234` outside of a working copy (does not require repository API).
  - Ran `arc browse T234` in a working copy, got task.
  - Ran `arc browse PATH` in a working copy.
  - Ran `arc browse nonpath` with and without `--force`.

Reviewers: csilvers, btrahan

Reviewed By: btrahan

Subscribers: epriestley, spicyj

Maniphest Tasks: T5781

Differential Revision: https://secure.phabricator.com/D10140
Add a --browse flag to `arc diff`, to open objects in a browser after creating them

Summary: Ref T5781. Add a flag to optionally open a web browser after creating a diff or revision.

Test Plan: Created //this revision// with `arc diff --browse` // !!! //

Reviewers: csilvers, btrahan

Reviewed By: btrahan

Subscribers: epriestley, spicyj

Maniphest Tasks: T5781

Differential Revision: https://secure.phabricator.com/D10141
Add a --browse flag to `arc todo`

Summary: Ref T5781. Like D10141, but for `arc todo`. Fixes T5787.

Test Plan: See T5787.

Reviewers: btrahan, csilvers

Reviewed By: csilvers

Subscribers: epriestley, spicyj

Maniphest Tasks: T5781, T5787

Differential Revision: https://secure.phabricator.com/D10142
Allow "arc browse <symbol>" to resolve symbolic commits

Summary: Ref T5781. This makes things like `arc browse master` work (but they open the commit, not a revision).

Test Plan: Ran `arc browse master`.

Reviewers: csilvers, btrahan

Reviewed By: btrahan

Subscribers: epriestley, spicyj

Maniphest Tasks: T5781

Differential Revision: https://secure.phabricator.com/D10143
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then