RepoProject: read the "upstream" attribute of a project

RepoProject: read the "upstream" attribute of a project

The manifest spec [1] defines the "upstream" attribute: "name of the
git ref in which a sha1 can be found", when the revision is a
sha1. The parser is ignoring it, but RepoCommand could use it to
populate the "ref=" field of pinned submodules.

Parse the value and store it in the RepoProject.

RepoProject is public API and the current constructors are not
telescopic, so we cannot just add a new constructor with an extra
argument. Use plain getter/setters.j

[1] https://gerrit.googlesource.com/git-repo/+/master/docs/manifest-format.md#Element-project

Change-Id: Ia50b85b95bfd3710f9fbda2050be5950dd686941