perf(gui): coalesce diff refresh after staging

[HiFiveJazz]

Summary

This PR avoids eagerly refreshing the diff immediately after optimistic stage/unstage updates.

The existing stage/unstage flow already does a pretty good job of doing status-only refresh after the background Git operation completes, and that refresh schedules a diff update using authoritative repository state. However, setting needs_diff_refresh before the Git operation completes causes the same mutable diff to be generated twice, which is unnecessary

This removes those three eager diff invalidations for:

• individual file stage/unstage
• directory stage/unstage
• stage-all/unstage-all

The optimistic file-status update remains unchanged, so staging still updates the file list immediately.

Performance

Measured with strace -f -e trace=execve, with auto-refresh and auto-fetch disabled.

Stage-all + unstage-all:

• Before: 48 Git subprocesses
• After: 44 Git subprocesses
• Reduction: 8.3%

Single-file stage + unstage:

• Before: 44 Git subprocesses
• After: 42 Git subprocesses
• Reduction: 4.5%

The removed subprocesses are the redundant diff previews issued before the authoritative status refresh. The later diff refresh after git status --porcelain -uall remains. I tested this using cargo test and saw no regressions.