Merge branch 'th/difftool-diffall'

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