Merge branch 'ex/deprecate-empty-pathspec-as-match-all'

The final step to make an empty string as a pathspec element illegal. We started this by first deprecating and warning a pathspec that has such an element in 2.11 (Nov 2016). Hopefully we can merge this down to the 'master' by the end of the year? A deprecation warning period that is about 1 year does not sound too bad. * ex/deprecate-empty-pathspec-as-match-all: pathspec: die on empty strings as pathspec t0027: do not use an empty string as a pathspec element