Merge branch 'bp/checkout-new-branch-optim'

Merge branch 'bp/checkout-new-branch-optim'

"git checkout -b <new> [HEAD]" to create a new branch from the
current commit and check it out ought to be a no-op in the index
and the working tree in normal cases, but there are corner cases
that do require updates to the index and the working tree.  Running
it immediately after "git clone --no-checkout" is one of these
cases that an earlier optimization kicked in incorrectly, which has
been fixed.

* bp/checkout-new-branch-optim:
  checkout: fix regression in checkout -b on intitial checkout
  checkout: add test demonstrating regression with checkout -b on initial commit