Merge branch 'nd/checkout-disambiguation' into maint

"git checkout <word>" does not follow the usual disambiguation
rules when the <word> can be both a rev and a path, to allow
checking out a branch 'foo' in a project that happens to have a
file 'foo' in the working tree without having to disambiguate.
This was poorly documented and the check was incorrect when the
command was run from a subdirectory.

* nd/checkout-disambiguation:
  checkout: fix ambiguity check in subdir
  checkout.txt: document a common case that ignores ambiguation rules
  checkout: add some spaces between code and comment

diff --cc builtin/checkout.c
Simple merge