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

diff --cc builtin/checkout.c
Simple merge

diff --cc t/t9902-completion.sh
Simple merge