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>

modules/setting/config.go

@@ -49,6 +49,7 @@ func DefaultOpenWithEditorApps() OpenWithEditorAppsType {
 
 type RepositoryStruct struct {
 	OpenWithEditorApps *config.Value[OpenWithEditorAppsType]
+	GitGuideRemoteName *config.Value[string]
 }
 
 type ConfigStruct struct {
@@ -70,6 +71,7 @@ func initDefaultConfig() {
 		},
 		Repository: &RepositoryStruct{
 			OpenWithEditorApps: config.ValueJSON[OpenWithEditorAppsType]("repository.open-with.editor-apps"),
+			GitGuideRemoteName: config.ValueJSON[string]("repository.git-guide-remote-name").WithDefault("origin"),
 		},
 	}
 }