Merge branch 'nd/checkout-disambiguation'

"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