As well as checking for the presence of tags it should also check that the revisions referred to are in the ancestry of the branch

When "uncommit" is done then tags that refer to those revisions that are removed
from the branch are not removed, so just looking up a tag doesn't mean that it should
be used. It should be checked that the revision is present in the ancestry of the branch
as well.

If a tag is found, but is not present in the ancestry it should be ok to just move it.

E.g. "merge-upstream; uncommit; merge-upstream" will currently not be allowed,
as the tag set by the first is still present.