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