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/api/v1/repo/pull.go

@@ -1591,6 +1591,7 @@ func GetPullRequestFiles(ctx *context.APIContext) {
 	maxLines := setting.Git.MaxGitDiffLines
 
 	// FIXME: If there are too many files in the repo, may cause some unpredictable issues.
+	// FIXME: it doesn't need to call "GetDiff" to do various parsing and highlighting
 	diff, err := gitdiff.GetDiff(ctx, baseGitRepo,
 		&gitdiff.DiffOptions{
 			BeforeCommitID:     startCommitID,
@@ -1606,9 +1607,14 @@ func GetPullRequestFiles(ctx *context.APIContext) {
 		return
 	}
 
+	diffShortStat, err := gitdiff.GetDiffShortStat(baseGitRepo, startCommitID, endCommitID)
+	if err != nil {
+		ctx.APIErrorInternal(err)
+		return
+	}
 	listOptions := utils.GetListOptions(ctx)
 
-	totalNumberOfFiles := diff.NumFiles
+	totalNumberOfFiles := diffShortStat.NumFiles
 	totalNumberOfPages := int(math.Ceil(float64(totalNumberOfFiles) / float64(listOptions.PageSize)))
 
 	start, limit := listOptions.GetSkipTake()