With this change, and running 'git auto-svn' whenever starting work on a
new upstream branch, much git-svn confusion and mayhem can be avoided.
For example:

$ git checkout -b drover_2403 branch-heads/2403
$ git svn info --url
  svn://svn.chromium.org/chrome-internal/trunk/tools/build
  # git-svn still thinks it's dealing with trunk.
$ git auto-svn
...
$ git svn info --url
  svn://svn.chromium.org/chrome-internal/branches/2403/tools/build
  # That's better.

R=agable@chromium.org, amineer@chromium.org

Review URL: https://codereview.chromium.org/1340733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296667 0039d316-1c4b-4281-b951-d872f2087c98