Add help for when you update a submodule by accident

[jyn514]

Ideally, I think the instructions to fix this would be a standalone tool you could run automatically. This seems like a good intermediate step, though.

[chenyukang]

@jyn514
I was annoyed by this for some time, now I use this justfile for just to rebase or sync master:

old_branch := `git rev-parse --abbrev-ref HEAD`
rebm:
        git stash
        git checkout up-master
        git pull
        git submodule sync
        git submodule update
        git stash
        git checkout {{old_branch}}
        git rebase up-master

sync:
        git stash
        git checkout up-master
        git pull
        git submodule sync
        git submodule update

Then only run j rebm to to rebase master.
I'm not sure whether we can provide some handy functions to help developers, but I'm sure for those devs who don't touch submodules, it will save a lot of time to fix submodule conflicts.

[jyn514]

Leaving comments on closed PRs is not a good way to contact me, I have too many github notifications to read them all.

It sounds like you have a suggestion to improve this section :) feel free to open either an issue or a PR.