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)