Merge branch 'jk/add-i-use-pathspecs' into next

Merge branch 'jk/add-i-use-pathspecs' into next

"git add -p <pathspec>" unnecessarily expanded the pathspec to a
list of individual files that matches the pathspec by running "git
ls-files <pathspec>", before feeding it to "git diff-index" to see
which paths have changes, because historically the pathspec
language supported by "diff-index" was weaker.  These days they are
equivalent and there is no reason to internally expand it.  This
helps both performance and avoids command line argument limit on
some platforms.

* jk/add-i-use-pathspecs:
  add--interactive: do not expand pathspecs with ls-files