Importer incorrectly detects untagged pushes

The importer currently considers a revision an untagged push if it brings no
changes vs. an import of the upload it is considering.

This is incorrect when the upload contains all changes present in the branch
but adds further changes.

The correct check is to compare the current tip to the test import of the new
upload, either via iter_changes (only considering content changes and
adds/deletes/renames/executability etc.), or by merging the test import in
the current tip and checking for a resulting empty diff.

Further, if the above detects that there are changes, it should check for
whether the merge of the current tip in to the test import brings in any
changes. If it does it should propose them for merge, and squash them
out of the history. If it doesn't we could preserve them in the history and
not propose a merge, as the content is all already included.