Improve Git support: safer workflows, reliable refresh and staged diffs

[hendrikmennen]

Summary

Improve Git support across commit/staging, diffs, branch and remote workflows, background refresh, and cloning.

• Fix the Commit Staged menu action staging all changes; validate messages and prevent Commit All from silently resolving conflicts.
• Separate staged (HEAD to index) and unstaged (index to working tree) comparisons, support initial commits/untracked files, and refresh comparisons on atomic index replacements.
• Confirm destructive actions, preserve staged edits during individual discard, and protect untracked files during hard reset.
• Preserve nested remote branch names, confirm branch deletion, and configure upstream only after successful publication.
• Serialize Git operations, keep pull/push atomic during sync, guarantee cleanup, and avoid stale ahead/behind counts suppressing pushes.
• Separate fetch and polling timers, scan status off the UI thread, dispose repository handles, and keep background fetch quiet and retryable.
• Complete clone-to-folder workflow with destination checks and cancellation; bound login attempts and handle cancelled identity dialogs.
• Open HEAD snapshots separately from working-tree editors and correct combined status badges and conflict counts.

Validation

• 40 regression tests pass in OneWare.SourceControl.UnitTests using temporary repositories and local bare remotes.
• Source-control module builds successfully.
• Desktop application builds successfully (existing warnings in unrelated modules).
• git diff --check passes.

Manual checks still needed

• Desktop click-through checks, particularly project switching during fetch and simultaneous polling/auto-fetch.
• Interactive authentication, private-repository pull/push, rejected pushes, and login cancellation.
• Clone cancellation and open-as-folder UI.

Workflow documentation and the smoke-check checklist are in docs/SourceControl.md. This PR does not add stash, rebase, history browsing, or new authentication providers.