Merge branch 'js/difftool-builtin'

Rewrite a scripted porcelain "git difftool" in C.

* js/difftool-builtin:
  difftool: hack around -Wzero-length-format warning
  difftool: retire the scripted version
  difftool: implement the functionality in the builtin
  difftool: add a skeleton for the upcoming builtin

diff --cc Makefile
Simple merge

diff --cc contrib/examples/git-difftool.perl
Simple merge

diff --cc git.c
Simple merge

diff --cc t/t7800-difftool.sh
Simple merge