Merge branch 'js/builtin-add-i'

The beginning of rewriting "git add -i" in C.

* js/builtin-add-i:
  built-in add -i: implement the `help` command
  built-in add -i: use color in the main loop
  built-in add -i: support `?` (prompt help)
  built-in add -i: show unique prefixes of the commands
  built-in add -i: implement the main loop
  built-in add -i: color the header in the `status` command
  built-in add -i: implement the `status` command
  diff: export diffstat interface
  Start to implement a built-in version of `git add --interactive`

diff --cc Makefile
Simple merge

diff --cc diff.c
Simple merge