Change project.revision to revisionExpr and revisionId

The revisionExpr field now holds an expression from the manifest,
such as "refs/heads/master", while revisionId holds the current
commit-ish SHA-1 of the revisionExpr.  Currently that is only
filled in if the manifest points directly to a SHA-1.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/subcmds/init.py b/subcmds/init.py
index 0d0fcd0..5ba4d79 100644
--- a/subcmds/init.py
+++ b/subcmds/init.py
@@ -113,12 +113,12 @@
       m._InitGitDir()
 
       if opt.manifest_branch:
-        m.revision = opt.manifest_branch
+        m.revisionExpr = opt.manifest_branch
       else:
-        m.revision = 'refs/heads/master'
+        m.revisionExpr = 'refs/heads/master'
     else:
       if opt.manifest_branch:
-        m.revision = opt.manifest_branch
+        m.revisionExpr = opt.manifest_branch
       else:
         m.PreSync()