RFC: Allow to create multiple changeset specs in a single repository

[mrnugget]

https://github.com/sourcegraph/sourcegraph/issues/14970 needs an RFC.

The goal for this ticket is to write an RFC that describes the current status of "multiple changeset specs in a single repository", how we could get it working, what that would look like in a campaign spec, what it would take on the backend.

[mrnugget]

Update since I'm close to the end of the time I set aside for this this sprint.

RFC is here: RFC 265 WIP: Multiple changesets per repository

Status: I think the problem description is complete, but I don't have any solutions yet. I'm actively and hopefully unconsciously trying to come up with something. The good news is that the backend has no problems with multiple changesets in the same repository, I could confirm that. That means we need to find a solution for producing multiple diffs in src-cli and how to define that in a campaign spec.

[mrnugget]

I just marked RFC 265: Multiple changesets per repository as ready for review to kick off the discussion as early as possible.

I implemented a working solution of my proposal in src-cli here: sourcegraph/src-cli#398 But before I document it and/or release it and/or get reviews on it, I want to start the review process of the RFC to, well, request comments (and questions, and ideas, and proposals, etc.)

[mrnugget]

I'm marking this ticket as done, because I think we've reached a "milestone" with this RFC:

• RFC is ready for review.
• RFC contains a fleshed-out version of 1 of 2 possible solutions.
• PR that implements this is ready to be reviewed.

We, as a team, still haven't made a decision as to which solution we should properly implement and ship, but I'd rather create a new ticket for this than to keep on moving this ticket from sprint to sprint.

cc @sourcegraph/campaigns