Properly URL encode branches in arc browse --branch ...

Summary: Fixes T10511. If you arc browse --branch x/y/z, we do not encode the URI properly.

Test Plan:
Ran arc browse --branch x/y/z/ something.c.

Before, got an error about "x" does not exist. This is wrong; the error should be about "x/y/z".

After, got the proper error:

Reviewers: chad, avivey

Reviewed By: avivey

Maniphest Tasks: T10511

Differential Revision: https://secure.phabricator.com/D15397