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/compare.go

@@ -624,14 +624,19 @@ func PrepareCompareDiff(
 			MaxFiles:           maxFiles,
 			WhitespaceBehavior: whitespaceBehavior,
 			DirectComparison:   ci.DirectComparison,
-			FileOnly:           fileOnly,
 		}, ctx.FormStrings("files")...)
 	if err != nil {
-		ctx.ServerError("GetDiffRangeWithWhitespaceBehavior", err)
+		ctx.ServerError("GetDiff", err)
 		return false
 	}
+	diffShortStat, err := gitdiff.GetDiffShortStat(ci.HeadGitRepo, beforeCommitID, headCommitID)
+	if err != nil {
+		ctx.ServerError("GetDiffShortStat", err)
+		return false
+	}
+	ctx.Data["DiffShortStat"] = diffShortStat
 	ctx.Data["Diff"] = diff
-	ctx.Data["DiffNotAvailable"] = diff.NumFiles == 0
+	ctx.Data["DiffNotAvailable"] = diffShortStat.NumFiles == 0
 
 	if !fileOnly {
 		diffTree, err := gitdiff.GetDiffTree(ctx, ci.HeadGitRepo, false, beforeCommitID, headCommitID)