Add a way to override the revision of an <extend-project>

This change adds support for the 'revision' attribute in
<extend-project>. This avoids the need to perform a <remove-project>
followed by a <project> in local manifests.

Change-Id: Id2834fcfc1ae0d74b3347bed3618f250bf696b1f

docs/manifest-format.txt

@@ -86,6 +86,7 @@ following DTD:
     <!ATTLIST extend-project name CDATA #REQUIRED>
     <!ATTLIST extend-project path CDATA #IMPLIED>
     <!ATTLIST extend-project groups CDATA #IMPLIED>
+    <!ATTLIST extend-project revision CDATA #IMPLIED>
 
     <!ELEMENT remove-project EMPTY>
     <!ATTLIST remove-project name  CDATA #REQUIRED>
@@ -300,6 +301,9 @@ at the specified path, rather than all projects with the given name.
 Attribute `groups`: List of additional groups to which this project
 belongs.  Same syntax as the corresponding element of `project`.
 
+Attribute `revision`: If specified, overrides the revision of the original
+project.  Same syntax as the corresponding element of `project`.
+
 Element annotation
 ------------------
 

manifest_xml.py

@@ -571,12 +571,15 @@ class XmlManifest(object):
         groups = node.getAttribute('groups')
         if groups:
           groups = self._ParseGroups(groups)
+        revision = node.getAttribute('revision')
 
         for p in self._projects[name]:
           if path and p.relpath != path:
             continue
           if groups:
             p.groups.extend(groups)
+          if revision:
+            p.revisionExpr = revision
       if node.nodeName == 'repo-hooks':
         # Get the name of the project and the (space-separated) list of enabled.
         repo_hooks_project = self._reqatt(node, 'in-project')