* When aborting a failed rebase, don't cascade errors if the abort itself
    fails.
  * When starting a rebase-update cycle, cd to the root of the repo. This avoids
    an issue when you run rebase-update from inside of a branch which adds a
    new folder.

R=agable@chromium.org
BUG=499031

Review URL: https://codereview.chromium.org/1180673003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295637 0039d316-1c4b-4281-b951-d872f2087c98