Use configurable remote name for git commands (#35172)

Closes #19403, and makes it possible to use any remote name in code
snippets for an empty repository and pull request.
This change is very helpful to me, because I always use different name
for my gitea remote.

Uses setting config module to store the value. Default is `origin` for
backward compatibility.

### Screenshots
<details>
<summary>Empty repo</summary>
<img width="791" height="398" alt="image"
src="https://github.com/user-attachments/assets/7214053d-a8dd-4e77-8c9d-78936d9859e0"
/>
</details>

<details>
<summary>Pull Request</summary>
<img width="591" height="452" alt="image"
src="https://github.com/user-attachments/assets/ebc3d25c-5d6d-481d-819d-9706af3c5594"
/>
</details>

<details>
<summary>Settings page</summary>
<img width="1438" height="839" alt="image"
src="https://github.com/user-attachments/assets/d92bfa2c-7adc-4efe-95fa-0c55ad13b3f5"
/>
</details>

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

options/locale/locale_en-US.ini

@@ -3425,6 +3425,7 @@ config.picture_service = Picture Service
 config.disable_gravatar = Disable Gravatar
 config.enable_federated_avatar = Enable Federated Avatars
 config.open_with_editor_app_help = The "Open with" editors for the clone menu. If left empty, the default will be used. Expand to see the default.
+config.git_guide_remote_name = Repository remote name for git commands in the guide
 
 config.git_config = Git Configuration
 config.git_disable_diff_highlight = Disable Diff Syntax Highlight