add submodule diff links (#33097)

This adds links to submodules in diffs, similar to the existing link
when viewing a repo at a specific commit. It does this by expanding diff
parsing to recognize changes to submodules, and find the specific refs
that are added, deleted or changed.

Related #25888

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/commit_info.go

@@ -7,5 +7,5 @@ package git
 type CommitInfo struct {
 	Entry         *TreeEntry
 	Commit        *Commit
-	SubModuleFile *CommitSubModuleFile
+	SubmoduleFile *CommitSubmoduleFile
 }