[RFC] new subcommand: git sync

Hi folks,

I created a new "git sync" sub-command a few months ago to deal with the pull request workflow.

Its goals are to:
 - keep all configured branches synchronized with the remotes (--set-upstream)
 - do not touch your wip feature branches (which has diverged from upstream)
 - prune the remotes

As I use it on a daily basis, to synchronize the remotes and then be able to quickly rebase my pull requests. I think it's worth sharing. What do you think?

For now it is a simple shell script available here: https://github.com/jmlemetayer/one-time-setup/blob/main/git-sync

If you think it's a good idea, I'll propose a series of patches with the new sub-command, the manual page and the associated tests.

Best regards, Jean-Marie Lemetayer