Merge branch 'am/checkout-file-and-ref-ref-ambiguity'

"git checkout X" did not correctly fail when X is not a local branch but could name more than one remote-tracking branches (i.e. to be dwimmed as the starting point to create a corresponding local branch), which has been corrected. * am/checkout-file-and-ref-ref-ambiguity: checkout: don't revert file on ambiguous tracking branches parse_branchname_arg(): extract part as new function