Merge branch 'js/use-builtin-add-i'

"git add -i" was rewritten in C some time ago and has been in
testing; the reimplementation is now exposed to general public by
default.

* js/use-builtin-add-i:
  add -i: default to the built-in implementation
  t2016: require the PERL prereq only when necessary

diff --cc builtin/add.c
Simple merge

diff --cc ci/run-build-and-tests.sh
Simple merge

diff --cc t/README
Simple merge