An empty string used as a pathspec element has always meant
'everything matches', but it is too easy to write a script that
finds a path to remove in $path and run 'git rm "$paht"', which
ends up removing everything.  Start warning about this use of an
empty string used for 'everything matches' and ask users to use a
more explicit '.' for that instead.

The hope is that existing users will not mind this change, and
eventually the warning can be turned into a hard error, upgrading
the deprecation into removal of this (mis)feature.

* ex/deprecate-empty-pathspec-as-match-all:
  pathspec: warn on empty strings as pathspec