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

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

diff --cc git-add--interactive.perl
Simple merge

diff --cc t/t3701-add-interactive.sh
Simple merge