whatchanged: customize diff-tree output

whatchanged: customize diff-tree output

This allows the configuration item whatchanged.difftree to
control the output from git-whatchanged command.  For example:

	[whatchanged]
        	difftree = --pretty=fuller --name-status -M

does rename detection, shows the commit header in "fuller"
format and lists affected pathnames and the kind of changes to
them.

When no such configuration item exists, the output format
defaults to "--pretty -M --abbrev".

Signed-off-by: Junio C Hamano <junkio@cox.net>