figure out target branch dynamically

[whenceforth]

Newer projects tend to merge pull requests into 'main'. Older projects tend to merge pull requests into 'master'. Other projects may do something else entirely.

The current code uses a config var. It would be nicer to be able get it dynamically so the config is not needed except in unusual cases. This looks like a good way to get it:

$ git branch -r --list 'origin/HEAD'
   origin/HEAD -> origin/main

• Don't do the dynamic lookup if the target is configured
• Comment out the target from the default config and add info for why it's optional, etc.
• Make sure it works for repos that don't have a remote.

I am continuing to assume the upstream is 'origin' Handling multiple remotes is something I don't need yet. Will look into that if the need arises.