Merge branch 'js/add-p-in-c'

The effort to move "git-add--interactive" to C continues.

* js/add-p-in-c:
  built-in add -p: show helpful hint when nothing can be staged
  built-in add -p: only show the applicable parts of the help text
  built-in add -p: implement the 'q' ("quit") command
  built-in add -p: implement the '/' ("search regex") command
  built-in add -p: implement the 'g' ("goto") command
  built-in add -p: implement hunk editing
  strbuf: add a helper function to call the editor "on an strbuf"
  built-in add -p: coalesce hunks after splitting them
  built-in add -p: implement the hunk splitting feature
  built-in add -p: show different prompts for mode changes and deletions
  built-in app -p: allow selecting a mode change as a "hunk"
  built-in add -p: handle deleted empty files
  built-in add -p: support multi-file diffs
  built-in add -p: offer a helpful error message when hunk navigation failed
  built-in add -p: color the prompt and the help text
  built-in add -p: adjust hunk headers as needed
  built-in add -p: show colored hunks by default
  built-in add -i: wire up the new C code for the `patch` command
  built-in add -i: start implementing the `patch` functionality in C

diff --cc Makefile
Simple merge

diff --cc builtin/add.c
Simple merge