perf(git): speed up activity top authors and subdirectory listings (#39526)

Speeds up two slow git paths. Results match `main` apart
from the `log.follow` fix.

- Activity top authors skip `--numstat` and an unused `rev-list
--count`, they only need names and emails
- Subdirectory listings pass only the directory as pathspec, which
already matches all its entries
- Directory listings pass `--no-follow` to `git log`, a configured
`log.follow` disabled parent rewriting and gave wrong last commits

| Benchmark | main | PR | Change |
|---|--:|--:|--:|
| Top authors, gitea, 1 month | 632 ms | 21 ms | -97% |
| Top authors, gitea, 1 year | 3216 ms | 68 ms | -98% |
| Top authors, tea | 62 ms | 12 ms | -80% |
| Listing, git `Documentation/technical` | 189 ms | 91 ms | -52% |
| Listing, gitea `options/license` | 329 ms | 208 ms | -37% |
| Listing, gitea `templates/repo` | 294 ms | 223 ms | -24% |

Tested with unit and sqlite integration tests in default and `gogit`
builds on git 2.25 and 2.56, and by comparing listing results and cache
writes with `main` on randomized histories and the gitea, tea and git
repos. Benchmarks are medians of 8 interleaved macOS runs.

models/activities/repo_activity.go

@@ -83,23 +83,17 @@ func GetActivityStats(ctx context.Context, repo *repo_model.Repository, timeFrom
 
 // GetActivityStatsTopAuthors returns top author stats for git commits for all branches
 func GetActivityStatsTopAuthors(ctx context.Context, repo *repo_model.Repository, timeFrom time.Time, count int) ([]*ActivityAuthorData, error) {
-	gitRepo, closer, err := git.RepositoryFromContextOrOpen(ctx, repo)
-	if err != nil {
-		return nil, fmt.Errorf("OpenRepository: %w", err)
-	}
-	defer closer.Close()
-
-	code, err := gitRepo.GetCodeActivityStats(ctx, timeFrom, "")
+	authors, err := git.GetCodeActivityAuthors(ctx, repo, timeFrom)
 	if err != nil {
 		return nil, fmt.Errorf("FillFromGit: %w", err)
 	}
-	if code.Authors == nil {
+	if authors == nil {
 		return nil, nil
 	}
 	users := make(map[int64]*ActivityAuthorData)
 	var unknownUserID int64
 	unknownUserAvatarLink := user_model.NewGhostUser().AvatarLink(ctx)
-	for _, v := range code.Authors {
+	for _, v := range authors {
 		if len(v.Email) == 0 {
 			continue
 		}