Show a better message for empty repositories and invalid branches

Summary:
Ref T1493.

• When viewing an invalid branch, show a "there is no such branch" message.
• When viewing an empty repository, show a "this repository is empty" message.

Test Plan:

• Viewed empty, bad branch, and nonempty in Git.
• Viewed empty, bad branch, and nonempty in Mercurial.
• Viewed empty and nonempty in Subversion.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T1493

Differential Revision: https://secure.phabricator.com/D9912