Link to tree views of submodules if possible (#33424)

This is a follow-up to https://github.com/go-gitea/gitea/pull/33097. 

When linking a submodule at a commit in either the repo view, or a diff
when adding a new submodule, link to the tree view of that submodules
intead of the individual commit. This shows the user the full tree,
instead of the diff of the commit.

This makes the assumption that the tree for a given SHA is at
`<repo_url>/tree/<sha>`. This URL format is supported by both Github &
Gitlab, but not Gitea. To fix this, add a redirect from
`<username>/<repo>/tree/<ref>` to `<username>/<repo>/src/<ref>`, so that
Gitea can support this URL structure.

services/gitdiff/submodule_test.go

@@ -230,7 +230,7 @@ func TestSubmoduleInfo(t *testing.T) {
 	assert.EqualValues(t, "name", sdi.SubmoduleRepoLinkHTML(ctx))
 
 	sdi.SubmoduleFile = git.NewCommitSubmoduleFile("https://github.com/owner/repo", "1234")
-	assert.EqualValues(t, `<a href="https://github.com/owner/repo/commit/1111">1111</a>`, sdi.CommitRefIDLinkHTML(ctx, "1111"))
+	assert.EqualValues(t, `<a href="https://github.com/owner/repo/tree/1111">1111</a>`, sdi.CommitRefIDLinkHTML(ctx, "1111"))
 	assert.EqualValues(t, `<a href="https://github.com/owner/repo/compare/aaaa...bbbb">aaaa...bbbb</a>`, sdi.CompareRefIDLinkHTML(ctx))
 	assert.EqualValues(t, `<a href="https://github.com/owner/repo">name</a>`, sdi.SubmoduleRepoLinkHTML(ctx))
 }