Merge branch 'mp/complete-paths'

Merge branch 'mp/complete-paths'

The completion script used to let the default completer to suggest
pathnames, which gave too many irrelevant choices (e.g. "git add"
would not want to add an unmodified path).  Teach it to use a more
git-aware logic to enumerate only relevant ones.

* mp/complete-paths:
  git-completion.bash: add support for path completion