Merge branch 'jk/add-i-diff-filter'

Merge branch 'jk/add-i-diff-filter'

The "interactive.diffFilter" used by "git add -i" must retain
one-to-one correspondence between its input and output, but it was
not enforced and caused end-user confusion.  We now at least make
sure the filtered result has the same number of lines as its input
to detect a broken filter.

* jk/add-i-diff-filter:
  add--interactive: detect bogus diffFilter output
  t3701: add a test for interactive.diffFilter