feat: support tracking stable branches

[bryango]

Closes #13.

This PR implements tracking for stable branches. It includes the following changes:

• Branches are slightly re-ordered (now it's staging > staging-next > master > ,,,) to better represent the staging workflow.
• PRs targeting the stable branches are now correctly tracked, including stable staging PRs like the one mentioned in Feature request: track stable-targeted PRs (like backports) against stable branches #13.
• This is enabled by looking at the head and base fields of the API json responses, held in the PullRequest struct. Adding these fields is technically a breaking change for the lib crate, but really no one should be constructing these responses by hand and the fields are clearly non-exhaustive, so we take the chance to add the #[non_exhaustive] marker to the relevant structs.
• Along the way, we add some pagination parameters in the API url for it to return less irrelevant information. This is a hack which may speed up the responses and its processing. This is the first commit and it's not exactly related to the others so it can be dropped independently if it feels too unnecessarily hacky.

[uncenter]

This looks good and seems to work! I appreciate your comments about the different changes you made in the PR comment - very helpful. We can leave the pagination parameters hack if you think it'll help perf, I doubt it would make it worse.

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@acid-bong does this match your expectations for this behavior?

[uncenter]

We can leave the pagination parameters hack if you think it'll help perf, I doubt it would make it worse.

For what it's worth some very sketchy tests seem to put this PR at 33% and 47% faster for similar recently merged PRs (2.23s and 2.35s on main, vs 1.49s and 1.23s on this branch). Small sample but it does seem like a good idea!

[acid-bong]

My internet is too slow today, not sure if i can test it (i really hope this comment gets sent)

[acid-bong]

[414871] [Backport staging-25.05] cpython: 3.13.3 -> 3.13.4; 3.12.10 -> 3.12.11; 3.11.12 -> 3.11.13; 3.10.17 -> 3.10.18
Merged 42 days 15 hours 22 minutes and 16 seconds ago (2025-06-14T02:35:11+00:00), 6 days 2 hours 47 minutes and 48 seconds after creation, into branch 'staging-25.05'.
staging-25.05: ✅
staging-next-25.05: ✅
release-25.05: ✅
nixpkgs-25.05-darwin: ✅
nixos-25.05-small: ✅
nixos-25.05: ✅

Now that's perfect, thx a lot

[bryango]

Push a reformat commit, should pass CI now 😆

[uncenter]

Sorry for the delay, I had some annoying issues with release-please and cargo/crates.io publishing. Gave up on most of it and published v0.5.0 locally which finally includes your changes here. Thanks so much! ❤️