diff --git a/subcmds/diff.py b/subcmds/diff.py
index e024714..f233f69 100644
--- a/subcmds/diff.py
+++ b/subcmds/diff.py
@@ -20,8 +20,21 @@
   helpSummary = "Show changes between commit and working tree"
   helpUsage = """
 %prog [<project>...]
+
+The -u option causes '%prog' to generate diff output with file paths
+relative to the repository root, so the output can be applied
+to the Unix 'patch' command.
 """
 
+  def _Options(self, p):
+    def cmd(option, opt_str, value, parser):
+      setattr(parser.values, option.dest, list(parser.rargs))
+      while parser.rargs:
+        del parser.rargs[0]
+    p.add_option('-u', '--absolute',
+                 dest='absolute', action='store_true',
+                 help='Paths are relative to the repository root')
+
   def Execute(self, opt, args):
     for project in self.GetProjects(args):
-      project.PrintWorkTreeDiff()
+      project.PrintWorkTreeDiff(opt.absolute)