This way no history is lost, neither what was exported to the remote,
or the history of changes that is imported from it. No complicated
correlation of two possibly very different histories is needed, just
record what we know and then git merge will do a good job.
Also, it notices when the remote tracking branch doesn't need to be updated,
and avoids doing anything, so noop remotes are super cheap.
The only catch here is that, since the commits generated for imports
from the remote don't have a stable date or author/committer, each
(non-noop) import generates different commits for the same imported
trees. So, when the imported remote tracking branch is merged into master
and then a change is imported again, there will be an extra series of
commits, which will get more and more expensive each time.
This seems to call for making stable commits for imports. Also that
seems a good idea to make importing in several repositories have the
same result.
