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>