Detect remote default branch change on upgrading

There is a trend among package repos of renaming git default branch name from "master" to "main". Currently there's no easy way to detect such changes during upgrading, as a consequence, those submodules would simply remain in the old "master" branch forever. It would be very useful if borg could provide such function (detect changes, or even automatically update them). On a side note, I think this issue is similar to the change of remote repo address (e.g. some package moved from github to gitlab). However, the change of remote repo address is more easily detected because usually it breaks git-fetch.