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_submodule.go

@@ -3,6 +3,10 @@
 
 package git
 
+type SubmoduleWebLink struct {
+	RepoWebLink, CommitWebLink string
+}
+
 // GetSubModules get all the submodules of current revision git tree
 func (c *Commit) GetSubModules() (*ObjectCache[*SubModule], error) {
 	if c.submoduleCache != nil {