difftool protocol support

[davidfstr]

git can be configured with a graphical difftool such that git difftool <diff args> can be used to launch a GUI diff tool such as gvc.

gvc, being a graphical diff tool for git, would make sense to support the difftool protocol. It is explicitly within gvc's feature scope.

Tasks:

• Support the difftool protocol
• Document how to configure gvc to use the difftool protocol, in a place that users of gvc are likely to discover

---

Example configuration, using bc (Beyond Compare) as a difftool, to show what it might look like to configure gvc as a difftool:

# ~/.gitconfig

[diff]
	tool = bc
[difftool]
	prompt = false
[difftool "bc"]
	cmd = bcomp \"$LOCAL\" \"$REMOTE\"
	trustExitCode = true
[difftool "Kaleidoscope"]
	cmd = ksdiff --partial-changeset --relative-path \"$MERGED\" -- \"$LOCAL\" \"$REMOTE\"