enhance(api): add GitHub-compatible /repos/{owner}/{repo}/commits/{ref} endpoint (#38770)

Gitea currently exposes the endpoint:
 
`GET /repos/{owner}/{repo}/git/commits/{sha}`
 
to retrieve a single commit. However, GitHub provides the equivalent
endpoint as:
 
`GET /repos/{owner}/{repo}/commits/{ref}`
 
Applications integrating with both GitHub and Gitea must implement
platform-specific logic to use different endpoints, reducing API
compatibility.
 
Fixes #38225

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

templates/swagger/v1-openapi3.generated.json

@@ -20470,7 +20470,7 @@
             "$ref": "#/components/responses/validationError"
           }
         },
-        "summary": "Get a single commit from a repository",
+        "summary": "Get a single commit from a repository, it has a GitHub-compatible alias \"/repos/{owner}/{repo}/commits/{ref}\"",
         "tags": [
           "repository"
         ]