Fix submodule checkout error when using sync-s option

When sync-s="true" option is used, the checkout of a submodule will try
to use the revision attribute of the parent project.

If this revision is a named reference, the checkout will fail if there
is no reference with this name in the submodule.

The proposed solution is to use the git commit id as revisionExpr for
submodules.

Change-Id: Ie8390a11957fd6a9c61289c6861d13cb3fa11678

project.py

@@ -1754,7 +1754,7 @@ class Project(object):
                            objdir=objdir,
                            worktree=worktree,
                            relpath=relpath,
-                           revisionExpr=self.revisionExpr,
+                           revisionExpr=rev,
                            revisionId=rev,
                            rebase=self.rebase,
                            groups=self.groups,