improve performance of diffs (#32393)

This has two major changes that significantly reduce the amount of work
done for large diffs:

* Kill a running git process when reaching the maximum number of files
in a diff, preventing it from processing the entire diff.
* When loading a diff with the URL param `file-only=true`, skip loading
stats. This speeds up loading both hidden files of a diff and sections
of a diff when clicking the "Show More" button.

A couple of minor things from profiling are also included:

* Reuse existing repo in `PrepareViewPullInfo` if head and base are the
same.

The performance impact is going to depend heavily on the individual diff
and the hardware it runs on, but when testing locally on a diff changing
100k+ lines over hundreds of files, I'm seeing a roughly 75% reduction
in time to load the result of "Show More"

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
(cherry picked from commit 7dcccc3bb19655a6f83dd495ffc332708d0c8678)

routers/web/repo/compare.go

@@ -611,6 +611,8 @@ func PrepareCompareDiff(
 		maxLines, maxFiles = -1, -1
 	}
 
+	fileOnly := ctx.FormBool("file-only")
+
 	diff, err := gitdiff.GetDiff(ctx, ci.HeadGitRepo,
 		&gitdiff.DiffOptions{
 			BeforeCommitID:     beforeCommitID,
@@ -621,6 +623,7 @@ func PrepareCompareDiff(
 			MaxFiles:           maxFiles,
 			WhitespaceBehavior: whitespaceBehavior,
 			DirectComparison:   ci.DirectComparison,
+			FileOnly:           fileOnly,
 		}, ctx.FormStrings("files")...)
 	if err != nil {
 		ctx.ServerError("GetDiffRangeWithWhitespaceBehavior", err)