feat(sync): pull from remote main branch

[arxanas]

• Closes git branchless sync --pull not pulling main branch #355 by updating the local main branch to point to the remote main branch (assuming that it has no local commits, otherwise see next point).
• Closes git sync should be more controllable #574 by rebasing the local main commits on top of the remote main branch.
• Closes git reword --force-rewrite on a public commit moves the remote-tracking branch. #516, since git-branchless no longer knows about remote branches, so it doesn't try to move them.
• Closes git-branchless CLI hangs at 100% CPU #555. I don't know exactly why, but the problem is caused by the remote main branch being the upstream of the local main branch; since we no longer use remote main branches, the issue doesn't happen anymore.
• Closes git sync should be more controllable #574. git sync will now rebase commits onto the local main branch, and won't rebase the local main branch on top of the remote main branch unless you pass --pull.

That's 5 of the 18 open bugs!