Make pull --rebase on an unborn branch do a checkout

Make pull --rebase on an unborn branch do a checkout

A merging pull on an unborn branch was already supported. But a
rebasing pull failed. If the user has pull.rebase = true in his
user config, the pull would try to rebase. Rebasing needs a parent
commit, though. Native git handles this case:

  git init
  git remote add origin <URI>
  git pull --rebase origin master

Check up front in PullCommand for the unborn head and just do a
checkout in this case. MergeCommand already has similar code.

Bug: 544965
Change-Id: I1277e1ac0b0364b4623fd791f3d6b07bd5f58fca
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>