"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