Display base correctly for git-svn in update history

Summary:
for a git repo which is using git-svn, we should show the
revisioni number. For example, it is show "svn+ssh" if the git url
starts with "svn+ssh". Show the svn revision number instead.

Before fix:
https://secure.phabricator.com/file/view/PHID-FILE-j5rogyqjkgifoxtpwzcu/

After fix:
https://secure.phabricator.com/file/view/PHID-FILE-4vsnptcjzuzuyj4zo25x/

Test Plan:

• verified a diff with git-svn worked
• verified a pure git diff still worked

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1344