cli: Refactor out get_fetch_remotes into git_util.rs

[essiene]

Similarly to git_fetch, this will be used in jj git sync, so
moving this out to maintain the same behaviour across fetch command and sync.

• Refactor out get_fetch_remotes to git_util.rs.
• inline the different get_*_remote* functions into get_fetch_remotes;
  The resulting function is still small enough to be easily readable.
• Move get_single_remote into git_util.rs and update call sites.
• Update use references

All tests still pass.

Issue: #1039

[yuja]

Oh, we can't use the original args.fetch? That makes args.fetch useless..

[yuja]

Maybe FetchRemoteArgs can be extracted instead of all-in FetchArgs, but I'm not pretty sure if that makes sense. Sorry for the noise.

[essiene]

No worries at all, I may have jumped the gun on the Refactor before finalizing how it will be used. For sync it turns out we need to fetch all branches for all configured remotes[1], so the help text will be different after all, and I don't need to flatten them in. I've reverted that last change, back to just passing FetchArgs{} around with branch, remotes and all_remotes as the fields.

[1] - https://docs.google.com/document/d/1K4j1xQPFxtINhyOyKuCMTZ_iNrw8g-5_giBFMGoLjaI/edit?tab=t.0#bookmark=id.y91cr92o9czf

[yuja]

Let's not add branch to the args struct then. It doesn't make sense to pass branch in to get_fetch_remotes(), and all_remotes to git_fetch().

It might also be simpler to just make both get_all_remotes() and get_default_fetch_remotes() public.

[essiene]

So I think I finally have something that looks descent:

• git_fetch takes FetchArgs; which only has fields for branch and remotes. This is useful since FetchArgs disambiguated the fields and makes it easier to know what goes where.
• get_fetch_remotes doesn't need FetchArgs and since remotes and all_remotes are different types completely, it doesn't look terrible if I just pass them in directly

wdyt?

[yuja]

Seems okay.

FWIW, I wonder if git_fetch() can actually be reused in "git sync". Suppose "git sync" does rebase based on the stats returned by git::fetch(), "git sync" will have to use lower-level API.

Regarding args.all_remotes, copy-pasting if args.all_remotes { ... } block wouldn't be so bad.