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

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