feature: add Compare with default base on current branch menu

[ivnnv]

Other local branches in the sidebar already have a one-click "Compare with HEAD" entry, but the current branch's right-click only offers the free-form "Compare with..." palette. Adds a "Compare with <default>" entry that opens the existing Compare window pre-filled with the repo's default base branch.

The default branch is resolved from refs/remotes/<remote>/HEAD (the same source GitHub/GitLab use), with the remote chosen via the same fallback order used elsewhere in the app: current branch's upstream remote → Settings.DefaultRemote → first remote. The entry is hidden when the remote HEAD is unset or when the resolved base equals the current branch (so it never shows "Compare master with master").

Test plan

• Right-click the current branch and confirm "Compare with <default>" appears, opens the existing Compare window pre-filled.

[love-linger]

Compare with HEAD is designed to allow quick comparison with the current branch (excluding uncommitted files - that's why we use HEAD instead of current branch name), and it has nothing to do with the default(main) branch.

I don't think SourceGit needs to integrate the feature of comparing with <remote>/HEAD:

• When fallback to <remote>/HEAD, it means that the <remote>/HEAD is not the upstream of current branch. Therefore, I do not think we need to provide an individual context menu entry for this purpose. If users need it, they can select any branch to be compared from the Compare with... context menu entry.
• User may confuse about that why non-current local branches have a context menu to compare with current branch's HEAD but the current local branch has a context menu to compare with remote's HEAD?

So, I pushed another commit that adds a quick context menu entry for all local branches to compare with its upstream