Improve graph readability and add a staged changes view for filtered branches

[10946009]

Summary / 摘要

This PR improves the readability of the git graph and adds a dedicated
staged changes view when a specific branch filter is active.

此 PR 改善了 git graph 的可讀性，並在選擇特定 branch 篩選時，新增專用的
staged changes 檢視畫面。

Changes / 變更內容

• keep vertical graph lines at branch crossings instead of rendering a
  cross glyph

• add the short commit hash column to the main graph list

• add a c changes view when a specific branch filter is active

• split the changes view into Added and Not added sections

• show a file list on the left and the selected diff on the right in a
  40/60 layout

• support Tab focus switching and diff scrolling in the changes view

• colorize change types and line counts in the changes panel

• 多分支交會時保留垂直線，不再使用交叉 glyph

• 在主畫面 commit 清單新增短版 commit hash 欄位

• 當選擇特定 branch 篩選時，可按 c 開啟 changes 檢視

• 將 changes 檢視分為 Added 與 Not added 兩個區塊

• 左側顯示檔案清單、右側顯示所選檔案 diff，版面比例為 40/60

• 在 changes 檢視中支援 Tab 切換焦點，以及 diff 捲動

• 在 changes 面板中加入變更類型與增減行數的顏色標示

Behavior / 行為說明

• pressing c only works when a specific branch filter is selected

• Added shows staged changes

• Not added shows unstaged and untracked changes

• the right pane shows the diff for the selected file from the correct
  staged or unstaged source

• 只有在選擇特定 branch 篩選時，按 c 才會開啟 changes 檢視

• Added 顯示已 git add 的 staged changes

• Not added 顯示尚未 add 的 unstaged 與 untracked changes

• 右側面板會依照所選檔案，顯示對應的 staged 或 unstaged diff