Fix XmlManifest.Save with remotes that have 'alias' set

When the alias attribute is set for a remote, the RemoteSpec attached to
a Project only contains the alias name used by git, not the original
name used in the manifest. But that's not enough information to
reconstruct the manifest, so save off the original manifest name as
another RemoteSpec parameter, only used to write the manifest out.

Bug: Issue 181
Bug: Issue 219
Change-Id: Id7417dfd6ce5572e4e5fe14f22924fdf088ca4f3