Follow file symlinks in the UI to their target (#28835)

Symlinks are followed when you click on a link next to an entry, either
until a file has been found or until we know that the link is dead.
When the link cannot be accessed, we fall back to the current behavior
of showing the document containing the target.

---------

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

modules/git/commit.go

@@ -20,7 +20,8 @@ import (
 
 // Commit represents a git commit.
 type Commit struct {
-	Tree
+	Tree // FIXME: bad design, this field can be nil if the commit is from "last commit cache"
+
 	ID            ObjectID // The ID of this commit object
 	Author        *Signature
 	Committer     *Signature