vg circularize tweaks

[faithokamoto]

Changelog Entry

To be copied to the draft changelog by merger:

• Smoother interface & errors for vg circularize

Description

I saw something that I could improve, and who's going to tell me no?

A collection of minor tweaks to vg circularize. It was implemented over a decade ago by #281 as a minor utility and seems to have been mostly left alone.

• If the --head ID was larger than the --tail ID, then nothing happened but that wasn't obvious. Now we print an explicit error.
• If --head/--tail were used, circularizing paths by name wasn't allowed. I don't really see why that restriction should exist so I removed it.
• Now --path can be used multiple times, just in case that's useful.
• Added a few unit tests.
• No need to have --describe here for listing path names when we already have a command purpose-built to list path names, i.e. vg paths --list.