git: Add support for git pull with rebase (#41117)

- Adds a new action `git::PullRebase` which adds `--rebase` in the final
command invoked by existing Git-Pull implementation.
- Includes the new action in "Fetch/Push" button in the Git Panel
(screenshot below)
- Adds key-binding for `git::PullRebase` in all three platforms,
following the existing key-binding patterns (`ctrl-g shift-down`)
- Update git docs to include the new action.

Sidenote: This is my first ever OSS contribution

Screenshot:

<img width="234" height="215" alt="image"
src="https://github.com/user-attachments/assets/713d068f-5ea5-444f-8d66-444ca65affc8"
/>

---

Release Notes:

- Git: Added `git: pull rebase` for running `git pull --rebase`.

assets/keymaps/default-linux.json

@@ -945,6 +945,7 @@
       "ctrl-g ctrl-g": "git::Fetch",
       "ctrl-g up": "git::Push",
       "ctrl-g down": "git::Pull",
+      "ctrl-g shift-down": "git::PullRebase",
       "ctrl-g shift-up": "git::ForcePush",
       "ctrl-g d": "git::Diff",
       "ctrl-g backspace": "git::RestoreTrackedFiles",