Add out-of-sync subcommand

[ErichDonGubler]

I'd really like to be able to triage local work for not yet being sync'd with upstream or push remotes. I want a subcommand called out-of-sync that will do it for me!

1. For each local branch, determine whether it is "out of sync":
   1. Get the hash of the local, upstream, and push refs' commits, where applicable, probably using something like git rev-parse.
   2. If there's only a local commit, then
2. Let all hashes associated with branches determined to be "out of sync" from the previous step be called $branches.
3. If $branches is empty, exit with a return status of 0.
4. Otherwise, run the equivalent of git-glimpse select $branches, and exit with a return status of 0.

This seems to overlap slightly with tools like brandon-rhodes/uncommitted, though they focus on crawling filesystems for repositories and indicating that something is out-of-sync. Here, we only focus on a single repository, like all other subcommands.