Support user supplied custom .repo/local_manifest.xml files

By creating a .repo/local_manifest.xml the user can add extra
projects into their client space, without touching the main
manifest script.

For example:

  $ cat .repo/local_manifest.xml
  <?xml version="1.0" encoding="UTF-8"?>
  <manifest>
   <project path="android-build"
            name="platform/build"
            remote="korg"
            revision="android-1.0" />
  </manifest>

Signed-off-by: Shawn O. Pearce <sop@google.com>

manifest.py

@@ -26,6 +26,7 @@ from remote import Remote
 from error import ManifestParseError
 
 MANIFEST_FILE_NAME = 'manifest.xml'
+LOCAL_MANIFEST_NAME = 'local_manifest.xml'
 
 class _Default(object):
   """Project defaults within the manifest."""
@@ -108,10 +109,20 @@ class Manifest(object):
 
   def _Load(self):
     if not self._loaded:
-      self._ParseManifest()
+      self._ParseManifest(True)
+
+      local = os.path.join(self.repodir, LOCAL_MANIFEST_NAME)
+      if os.path.exists(local):
+        try:
+          real = self.manifestFile
+          self.manifestFile = local
+          self._ParseManifest(False)
+        finally:
+          self.manifestFile = real
+
       self._loaded = True
 
-  def _ParseManifest(self):
+  def _ParseManifest(self, is_root_file):
     root = xml.dom.minidom.parse(self.manifestFile)
     if not root or not root.childNodes:
       raise ManifestParseError, \
@@ -124,6 +135,7 @@ class Manifest(object):
             "no <manifest> in %s" % \
             self.manifestFile
 
+    if is_root_file:
       self.branch = config.getAttribute('branch')
       if not self.branch:
         self.branch = 'default'