Merge branch 'en/zdiff3'

"Zealous diff3" style of merge conflict presentation has been added.

* en/zdiff3:
  update documentation for new zdiff3 conflictStyle
  xdiff: implement a zealous diff3, or "zdiff3"

diff --cc Documentation/git-checkout.txt
Simple merge

diff --cc builtin/checkout.c
Simple merge

diff --cc contrib/completion/git-completion.bash
Simple merge