arc browse: Support line number and reduce conduit call

Summary:
support common "filename:line number" format.
Also, remove redundent conduit call.

Test Plan: arc browse with and without :33 suffix.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6451