Recovery from having manually merged upstream's trunk for a snapshot release is too hard

Use case:

When preparing the 2.1.4 lucid SRU for bzr, I took the Debian packaging branch corresponding to the package currently in lucid (2.1.1), and did a merge-upstream for 2.1.2 and 2.1.3, then a plain merge from bzr/2.1 to catch up with tip to allow testing before 2.1.4 was cut. Now I want to merge-upstream the final version of 2.1.4....

$ bzr merge-upstream --version 2.1.4 ../bzr-2.1.4.tar.gz ../2.1 -r tag:bzr-2.1.4
Using distribution lucid
Using version string 2.1.4.
bzr: ERROR: Unable to find the tag for the previous upstream version, 2.1.4~bzr4877, in the branch: upstream-2.1.4~bzr4877

At this point, it's fairly unclear how to recover.

After poking around in the bzr-builddeb source, I modified the current_version determination for merge-upstream in cmds.py to use a hardcoded version. Perhaps we should add a --previous-upstream VERSION override option to merge-upstream, and enhance the "Unable to find the tag ..." message to advise either placing the appropriate tag, or using --previous-upstream to designate the actual upstream version which is last tagged correctly.