Merge branch 'jn/add-2.0-u-A-sans-pathspec' (early part)

Merge branch 'jn/add-2.0-u-A-sans-pathspec' (early part)

In Git 2.0, "git add -u" and "git add -A" without any pathspec will
update the index for all paths, including those outside the current
directory, making it more consistent with "commit -a".  To help the
migration pain, a warning is issued when the differences between the
current behaviour and the upcoming behaviour matters, i.e. when the
user has local changes outside the current directory.

* 'jn/add-2.0-u-A-sans-pathspec' (early part):
  add -A: only show pathless 'add -A' warning when changes exist outside cwd
  add -u: only show pathless 'add -u' warning when changes exist outside cwd
  add: make warn_pathless_add() a no-op after first call
  add: add a blank line at the end of pathless 'add [-u|-A]' warning
  add: make pathless 'add [-u|-A]' warning a file-global function