Rolls the two-directory-diff logic from diffall script (in contrib/) into
"git difftool" framework.
By Tim Henigan
* th/difftool-diffall:
difftool: print list of valid tools with '--tool-help'
difftool: teach difftool to handle directory diffs
difftool: eliminate setup_environment function
difftool: stop appending '.exe' to git
difftool: remove explicit change of PATH
difftool: exit(0) when usage is printed
difftool: add '--no-gui' option
difftool: parse options using Getopt::Long