Use correct README link to render the README (#23152) (#23264)

Backport #23152

`renderReadmeFile` needs `readmeTreelink` as parameter but gets
`treeLink`.
The values of them look like as following:
`treeLink`:  `/{OwnerName}/{RepoName}/src/branch/{BranchName}`
`readmeTreelink`:
`/{OwnerName}/{RepoName}/src/branch/{BranchName}/{ReadmeFileName}`

`path.Dir` in

https://github.com/go-gitea/gitea/blob/8540fc45b11eff9a73753ca139f8ea5c38509bf5/routers/web/repo/view.go#L316
should convert `readmeTreelink` into
`/{OwnerName}/{RepoName}/src/branch/{BranchName}` instead of the current
`/{OwnerName}/{RepoName}/src/branch`.

Fixes #23151

Co-authored-by: yp05327 <576951401@qq.com>
Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

tests/gitea-repositories-meta/user2/repo1.git/refs/heads/home-md-img-check

@@ -0,0 +1 @@
+78fb907e3a3309eae4fe8fef030874cebbf1cd5e

tests/integration/repo_test.go

@@ -256,3 +256,23 @@ func TestViewRepoDirectory(t *testing.T) {
 	assert.Zero(t, repoTopics.Length())
 	assert.Zero(t, repoSummary.Length())
 }
+
+func TestMarkDownImage(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	session := loginUser(t, "user2")
+
+	req := NewRequest(t, "GET", "/user2/repo1/src/branch/home-md-img-check")
+	resp := session.MakeRequest(t, req, http.StatusOK)
+
+	htmlDoc := NewHTMLParser(t, resp.Body)
+	_, exists := htmlDoc.doc.Find(`img[src="/user2/repo1/media/branch/home-md-img-check/test-fake-img.jpg"]`).Attr("src")
+	assert.True(t, exists, "Repo home page markdown image link check failed")
+
+	req = NewRequest(t, "GET", "/user2/repo1/src/branch/home-md-img-check/README.md")
+	resp = session.MakeRequest(t, req, http.StatusOK)
+
+	htmlDoc = NewHTMLParser(t, resp.Body)
+	_, exists = htmlDoc.doc.Find(`img[src="/user2/repo1/media/branch/home-md-img-check/test-fake-img.jpg"]`).Attr("src")
+	assert.True(t, exists, "Repo src page markdown image link check failed")
+}