Option for 'repo diff' to generate output suitable for 'patch' cmd

The -u option causes 'repo diff' to generate diff output
with file paths relative to the repository root,
so the output can be applied to the Unix 'patch' command.
The name '-u' was selected for convenience, because
both 'diff' and 'git diff' accept the option with the same name
to generate an 'unified diff' output suitable for 'patch' command.

Change-Id: I79c8356db4ed20ecaccc258b3ba139db76666fe0
Reviewed-on: https://gerrit-review.googlesource.com/34380
Reviewed-by: Shawn Pearce <sop@google.com>
Tested-by: Shawn Pearce <sop@google.com>

project.py

diff --git a/project.py b/project.py
index 946a6ac..303abe3 100644
--- a/project.py
+++ b/project.py
@@ -749,7 +749,7 @@
 
     return 'DIRTY'
 
-  def PrintWorkTreeDiff(self):
+  def PrintWorkTreeDiff(self, absolute_paths=False):
     """Prints the status of the repository to stdout.
     """
     out = DiffColoring(self.config)
@@ -757,6 +757,9 @@
     if out.is_on:
       cmd.append('--color')
     cmd.append(HEAD)
+    if absolute_paths:
+      cmd.append('--src-prefix=a/%s/' % self.relpath)
+      cmd.append('--dst-prefix=b/%s/' % self.relpath)
     cmd.append('--')
     p = GitCommand(self,
                    cmd,

subcmds/diff.py

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)