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`.

docs/src/git.md

@@ -179,6 +179,7 @@ When viewing files with changes, Zed displays diff hunks that can be expanded or
 | {#action git::Push}                       | {#kb git::Push}                       |
 | {#action git::ForcePush}                  | {#kb git::ForcePush}                  |
 | {#action git::Pull}                       | {#kb git::Pull}                       |
+| {#action git::PullRebase}                 | {#kb git::PullRebase}                 |
 | {#action git::Fetch}                      | {#kb git::Fetch}                      |
 | {#action git::Diff}                       | {#kb git::Diff}                       |
 | {#action git::Restore}                    | {#kb git::Restore}                    |