Simplify `vcs::find_heads` implementation

[szokeasaurusrex]

The current implementation is quite difficult to understand; much of the logic is being performed in find_matching_revs and find_matching_rev. It's very difficult to clearly see the "happy path," and I suspect we could greatly simplify the logic.

Let's first address getsentry/sentry-cli#2734. If we implement that change per my suggestion, I believe a decent portion of the logic in vcs::find_heads may become unnecessary, allowing further simplification.

Blocked by REPLAY-413

[linear[bot]]

CLI-166 Simplify `vcs::find_heads` implementation