Merge branch 'th/difftool-diffall'

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