Merge branch 'nd/checkout-dwim-fix'

"git checkout frotz" (without any double-dash) avoids ambiguity by making sure 'frotz' cannot be interpreted as a revision and as a path at the same time. This safety has been updated to check also a unique remote-tracking branch 'frotz' in a remote, when dwimming to create a local branch 'frotz' out of a remote-tracking branch 'frotz' from a remote. * nd/checkout-dwim-fix: checkout: disambiguate dwim tracking branches and local files