Merge branch 'jc/all-negative-pathspec'

Merge branch 'jc/all-negative-pathspec'

A git subcommand like "git add -p" spawns a separate git process
while relaying its command line arguments.  A pathspec with only
negative elements was mistakenly passed with an empty string, which
has been corrected.

* jc/all-negative-pathspec:
  pathspec: correct an empty string used as a pathspec element