[U-Boot] Attn Maintainers: git advise needed (how to fix messed up repo)

[Andy Fleming]

Ignoring most of the context for the moment, I think fixing your
problem requires:

Start in your current, broken branch:
1) git branch save_broken_branch #just in case
2) git rebase -i u-boot/master

Interactive rebase will show you a long list of commits that aren't
yet in the mainline, even the duplicates.
Delete all the commits which are duplicates.

Hopefully, it completes without conflicts.

3) git diff save_broken_branch..HEAD # Should return nothing

Hope this helps. If there is a difference, then there's a chance that
somehow the merging ran into issues which it resolved in odd ways.
It's possible some of those commits are empty?  Try git log -p to see
what's actually in all of those commits.

Andy