Fix PR creation via api between branches of same repo with head field namespaced (#26986)

Fix #20175

Current implementation of API does not allow creating pull requests
between branches of the same
repo when you specify *namespace* (owner of the repo) in `head` field in
http request body.

---

Although GitHub implementation of API allows performing such action and
since Gitea targeting
compatibility with GitHub API I see it as an appropriate change.

I'm proposing a fix to the described problem and test case which covers
this logic.

My use-case just in case:
https://github.com/go-gitea/gitea/issues/20175#issuecomment-1711283022

(cherry picked from commit ed02d1fab85c9b8206c0af84dcfc3792e61609cf)

routers/api/v1/repo/pull.go

@@ -1065,6 +1065,8 @@ func parseCompareInfo(ctx *context.APIContext, form api.CreatePullRequestOption)
 			return nil, nil, nil, nil, "", ""
 		}
 		headBranch = headInfos[1]
+		// The head repository can also point to the same repo
+		isSameRepo = ctx.Repo.Owner.ID == headUser.ID
 
 	} else {
 		ctx.NotFound()