Use git diff-tree for DiffFileTree on diff pages (#33514)

Modify Diff View FileTree to show all files

## Changes

* removes Show Status button on diff
* uses `git diff-tree` to generate the file tree for the diff
* doesn't reload the diff tree each time we load more files in the
preview
* selecting and unloaded file will keep loading until that file is
loaded
* removes `DiffFileList.vue` and "Show Stats" in diff options

## Open Questions

* selecting and unloaded file will keep loading until that file is
loaded. Is this behaviour okay? It matches what github does.

### Demo

In this demo I set `git.MAX_GIT_DIFF_FILES=1` in my `app.ini` to
demonstrate a worst case example. In most cases the behaviour isn't
nearly as jarring as we load a bunch of files at a time.


https://github.com/user-attachments/assets/72f29663-d6fc-472d-94fa-7fb5950c2836

---------

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

routers/web/repo/treelist.go

@@ -6,9 +6,11 @@ package repo
 import (
 	"net/http"
 
+	pull_model "code.gitea.io/gitea/models/pull"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/services/context"
+	"code.gitea.io/gitea/services/gitdiff"
 
 	"github.com/go-enry/go-enry/v2"
 )
@@ -52,3 +54,33 @@ func isExcludedEntry(entry *git.TreeEntry) bool {
 
 	return false
 }
+
+type FileDiffFile struct {
+	Name        string
+	NameHash    string
+	IsSubmodule bool
+	IsViewed    bool
+	Status      string
+}
+
+// transformDiffTreeForUI transforms a DiffTree into a slice of FileDiffFile for UI rendering
+// it also takes a map of file names to their viewed state, which is used to mark files as viewed
+func transformDiffTreeForUI(diffTree *gitdiff.DiffTree, filesViewedState map[string]pull_model.ViewedState) []FileDiffFile {
+	files := make([]FileDiffFile, 0, len(diffTree.Files))
+
+	for _, file := range diffTree.Files {
+		nameHash := git.HashFilePathForWebUI(file.HeadPath)
+		isSubmodule := file.HeadMode == git.EntryModeCommit
+		isViewed := filesViewedState[file.HeadPath] == pull_model.Viewed
+
+		files = append(files, FileDiffFile{
+			Name:        file.HeadPath,
+			NameHash:    nameHash,
+			IsSubmodule: isSubmodule,
+			IsViewed:    isViewed,
+			Status:      file.Status,
+		})
+	}
+
+	return files
+}