Fix a bug that left branches tracking a dead branch if both their parent
and grand-parent were left with no changes after a "rebase-update" step.

Given the following initial state:
  $ git map-branches -v
  origin/master
    a
      b
        c *        [ ahead 1 ]

without this patch, a "git rebase-update" on this tree state would
leave the branch "c" as tracking a non-existing branch "a":

  $ git recursive-rebase
  a up-to-date
  b up-to-date
  c up-to-date
  Reparented c to track a (was tracking b)
  Deleted branch b (was 448d1da).
  Deleted branch a (was 448d1da).
  $ git map-branches -v
  {a:GONE}
    c *

with the patch, we record that the branch "c" is tracking must be
updated twice and we end up in a state were "c" is correctly tracking
"origin/master":

  $ git recursive-rebase
  a up-to-date
  b up-to-date
  c up-to-date
  Reparented c to track origin/master (was tracking b)
  Deleted branch b (was 448d1da).
  Deleted branch a (was 448d1da).
  $ git map-branches -v
  origin/master
    c *            [ ahead 1 ]

BUG=456806

Review URL: https://codereview.chromium.org/1482753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297792 0039d316-1c4b-4281-b951-d872f2087c98