Decouple diff stats query from actual diffing (#33810)

The diff stats are no longer part of the diff generation.
Use `GetDiffShortStat` instead to get the total number of changed files,
added lines, and deleted lines.
As such, `gitdiff.GetDiff` can be simplified:
It should not do more than expected.

And do not run "git diff --shortstat" for pull list. Fix #31492

routers/web/repo/editor.go

@@ -423,7 +423,7 @@ func DiffPreviewPost(ctx *context.Context) {
 		return
 	}
 
-	if diff.NumFiles != 0 {
+	if len(diff.Files) != 0 {
 		ctx.Data["File"] = diff.Files[0]
 	}