rebase: use explicit "--" with checkout

rebase: use explicit "--" with checkout

In the case of a ref/pathname conflict, checkout will
already do the right thing and checkout the ref. However,
for a non-existant ref, this has two advantages:

  1. If a file with that pathname exists, rebase will
     refresh the file from the index and then rebase the
     current branch instead of producing an error.

  2. If no such file exists, the error message using an
     explicit "--" is better:

       # before
       $ git rebase -i origin bogus
       error: pathspec 'bogus' did not match any file(s) known to git.
       Could not checkout bogus

       # after
       $ git rebase -i origin bogus
       fatal: invalid reference: bogus
       Could not checkout bogus

The problems seem to be trigger-able only through "git
rebase -i", as regular git-rebase checks the validity of the
branch parameter as a ref very early on. However, it doesn't
hurt to be defensive.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>