support obtaining tag upstream-VERSION from other repository

[Scott Moser]

I was trying to build a daily build of lp:euca2ools , and kept hitting an issue that there was no tag upstream-1.3.1 in the branch (which make sense, as its not an ubuntu branch).

I was unable to do anything about this because
a.) I dont' have write access to lp:euca2ools
b.) even if I did, their release process was not solid, and there is no commit that actually represents what is in the 1.3.1 tarball.

The lp:ubuntu/euca2ools branch *does* have such a tag, and I could just use that if I could reference it.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: bzr-builder 0.7-0ubuntu1
ProcVersionSignature: Ubuntu 3.0-2.3-generic 3.0.0-rc4
Uname: Linux 3.0-2-generic x86_64
Architecture: amd64
Date: Thu Jul 14 16:50:14 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 10.04 "Lucid Lynx" - Beta amd64 (20100318)
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: bzr-builder
UpgradeStatus: Upgraded to oneiric on 2010-11-15 (240 days ago)