asyncgit: honour branch.sort config when listing branches

[Scolliq]

Closes #2024.

Why: get_branches_info hardcodes a lex-asc sort on the branch name, so users with branch.sort = -committerdate (or authordate, or -refname) in their git config see GitUI's branch list ignore that preference.

What changed: read branch.sort from the repo config and apply it. Supports refname, committerdate, authordate, each with the optional - prefix for descending. Anything else (including version:refname, which I left out for scope) falls back to the existing refname-ascending behaviour. Public API unchanged.

Tests: 6 parser unit tests covering recognised values, the descending prefix, whitespace, and unknown-falls-back. 2 integration tests with write_commit_file_at to pin the actual sort behaviour for -committerdate and the unset-config baseline.

cargo test -p asyncgit --lib sync::branch → 36 pass. cargo clippy -p asyncgit --all-features clean. cargo fmt --check clean.

Future scope: version:refname would need a semver-aware comparator — happy to follow up if there's interest.

[extrawurst]

Thanks for taking care of this. The only problem i see is that we make the remote branch list very hard to read if there are different remotes. I know this is exactly as git branch --remote lists them but there must be a better way.

I can see a better solution to do a hirarchical tree view first and only sort by this factor inside the same "folder". Is that something you could see yourself doing as a followup?

Right now this is likely still better than the status quo. Please add a changelog entry and rebase to current upstream master.

Also it would be great to update #2146 on top of this where we store the GUI selected sorting into the local git config and use that as the source of truth.

[extrawurst]

Git accepts a comma-separated list (e.g. -committerdate,refname). BranchSort::parse treats the whole string as one key, so -committerdate,refname falls through to the default. Either support it or document the subset.