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

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

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