git: Touch up amend UX (#35114)

Follow-up to #26114

- Ensure that the previous commit message is filled in when toggling on
amend mode from the context menu
- Fix keybinding flicker in context menu

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -872,8 +872,6 @@
       "tab": "git_panel::FocusEditor",
       "shift-tab": "git_panel::FocusEditor",
       "escape": "git_panel::ToggleFocus",
-      "ctrl-enter": "git::Commit",
-      "ctrl-shift-enter": "git::Amend",
       "alt-enter": "menu::SecondaryConfirm",
       "delete": ["git::RestoreFile", { "skip_prompt": false }],
       "backspace": ["git::RestoreFile", { "skip_prompt": false }],
@@ -910,7 +908,9 @@
       "ctrl-g backspace": "git::RestoreTrackedFiles",
       "ctrl-g shift-backspace": "git::TrashUntrackedFiles",
       "ctrl-space": "git::StageAll",
-      "ctrl-shift-space": "git::UnstageAll"
+      "ctrl-shift-space": "git::UnstageAll",
+      "ctrl-enter": "git::Commit",
+      "ctrl-shift-enter": "git::Amend"
     }
   },
   {