Merge branch 'mm/add-u-A-sans-pathspec'

Merge branch 'mm/add-u-A-sans-pathspec'

Forbid "git add -u" and "git add -A" without pathspec run from a
subdirectory, to train people to type "." (or ":/") to make the
choice of default does not matter.

* mm/add-u-A-sans-pathspec:
  add: warn when -u or -A is used without pathspec