Merge branch 'mm/add-p-split-error'

When "add--interactive" splits a hunk into two overlapping hunks
and then let the user choose only one, it sometimes feeds an
incorrect patch text to "git apply".  Add tests to demonstrate
this.

I have a slight suspicion that this may be $gmane/87202 coming back
and biting us (I seem to have said "let's run with this and see
what happens" back then).

* mm/add-p-split-error:
  stash -p: demonstrate failure of split with mixed y/n
  t3904-stash-patch: factor PERL prereq at the top of the file
  t3904-stash-patch: fix test description
  add -p: demonstrate failure when running 'edit' after a split
  t3701-add-interactive: simplify code