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>

routers/web/repo/commit.go

@@ -328,6 +328,7 @@ func Diff(ctx *context.Context) {
 		MaxLineCharacters:  setting.Git.MaxGitDiffLineCharacters,
 		MaxFiles:           maxFiles,
 		WhitespaceBehavior: gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)),
+		FileOnly:           fileOnly,
 	}, files...)
 	if err != nil {
 		ctx.NotFound("GetDiff", err)

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)

routers/web/repo/pull.go

@@ -395,12 +395,12 @@ func PrepareViewPullInfo(ctx *context.Context, issue *issues_model.Issue) *git.C
 	var headBranchSha string
 	// HeadRepo may be missing
 	if pull.HeadRepo != nil {
-		headGitRepo, err := gitrepo.OpenRepository(ctx, pull.HeadRepo)
+		headGitRepo, closer, err := gitrepo.RepositoryFromContextOrOpen(ctx, pull.HeadRepo)
 		if err != nil {
-			ctx.ServerError("OpenRepository", err)
+			ctx.ServerError("RepositoryFromContextOrOpen", err)
 			return nil
 		}
-		defer headGitRepo.Close()
+		defer closer.Close()
 
 		if pull.Flow == issues_model.PullRequestFlowGithub {
 			headBranchExist = headGitRepo.IsBranchExist(pull.HeadBranch)
@@ -747,6 +747,7 @@ func viewPullFiles(ctx *context.Context, specifiedStartCommit, specifiedEndCommi
 		MaxLineCharacters:  setting.Git.MaxGitDiffLineCharacters,
 		MaxFiles:           maxFiles,
 		WhitespaceBehavior: gitdiff.GetWhitespaceFlag(ctx.Data["WhitespaceBehavior"].(string)),
+		FileOnly:           fileOnly,
 	}
 
 	if !willShowSpecifiedCommit {