Explicitly pass "--diff-cmd diff" to "svn diff", for users who have set some other diff tool.

scm.py

Index: scm.py
===================================================================
--- scm.py	(revision 177805)
+++ scm.py	(working copy)
@@ -803,7 +803,8 @@
   @staticmethod
   def _DiffItemInternal(filename, cwd, info, bogus_dir, full_move, revision):
     """Grabs the diff data."""
-    command = ["diff", "--config-dir", bogus_dir, filename]
+    command = ["diff", "--diff-cmd", "diff", "--config-dir", bogus_dir,
+               filename]
     if revision:
       command.extend(['--revision', revision])
     data = None
@@ -915,7 +916,8 @@
               # revision the file was deleted.
               srcinfo = {'Revision': rev}
             if (srcinfo.get('Revision') != rev and
-                SVN.Capture(['diff', '-r', '%d:head' % rev, srcurl], cwd)):
+                SVN.Capture(['diff', '--diff-cmd', 'diff', '-r',
+                             '%d:head' % rev, srcurl], cwd)):
               metaheaders.append("#$ svn cp -r %d %s %s "
                                  "### WARNING: note non-trunk copy\n" %
                                  (rev, src, filename))