Use GNU diff instead of Solaris diff.

Solaris diff doesn't understand the -N option, which is part of the default

option set.

This is a very Solaris-specific patch, and should not be passed upstream.

diff --git a/hgext/extdiff.py b/hgext/extdiff.py

--- a/hgext/extdiff.py

+++ b/hgext/extdiff.py

@@ -282,7 +282,7 @@ def extdiff(ui, repo, *pats, **opts):

     program = opts.get('program')

     option = opts.get('option')

     if not program:

-        program = 'diff'

+        program = 'gdiff'

         option = option or ['-Npru']

     cmdline = ' '.join(map(util.shellquote, [program] + option))

     return dodiff(ui, repo, cmdline, pats, opts)