fixes local svn branch name detection

Summary: Corrects relative vs absolute branch name when using 'arc commit'

Test Plan: 'arc commit' on a release branch gave an error before making the change (change was generated from 'branches/yyy' but working copy root is 'https://xxx/branches/yyy', now it does not.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6743