git sync: Get branch heads and fetch from remote

[essiene]

Summary

• Get branch pre-fetch heads
• Build candidates from prefetch heads
• Fetch from remotes
• Get branch post-fetch heads
• Rebase

Details

• For branch heads, we grab local branches matching the pattern
  specified in args.
• Pre-fetch heads query tx.base_repo() while post-fetch heads
  query tx.repo(); tx.repo() is the in memory Mutable repo which
  is updated after a fetch in a transaction.
• We fetch from all branches from all configured remotes.
  --all_remotes affects if we use git.fetch or all repo remotes.

Issue: #1039

[essiene]

I'm not sure if I should add unit tests tests for these or just add integration tests for jj git sync directly; I'm planning to do that in the later PRs.

[yuja]

I'm not sure what we're going to achieve, but my gut feeling is that this and the subsequent PRs duplicate git::import_refs()? I would instead try to extend git:import_refs() to return information needed to rebase commits.

And the core "git sync" function might be implemented in lib/src/git.rs.

[essiene]

I'm not sure what we're going to achieve, but my gut feeling is that this and the subsequent PRs duplicate git::import_refs()? I would instead try to extend git:import_refs() to return information needed to rebase commits.

And the core "git sync" function might be implemented in lib/src/git.rs.

Thanks for the suggestion. Based on this and comments from @martinvonz, I've gone to actually read git::import_some_refs() and I understand it better now.

Abandoning this PR now and will send a new one that changes jj-lib to introduce core functionality for sync to be built atop of.