Merge branch 'ab/retire-scripted-add-p'

Finally retire the scripted "git add -p/-i" implementation and have everybody use the one reimplemented in C. * ab/retire-scripted-add-p: docs & comments: replace mentions of "git-add--interactive.perl" add API: remove run_add_interactive() wrapper function add: remove "add.interactive.useBuiltin" & Perl "git add--interactive"