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.

modules/git/repo_stats_test.go

@@ -33,4 +33,8 @@ func TestRepository_GetCodeActivityStats(t *testing.T) {
 	assert.Equal(t, "tris.git@shoddynet.org", code.Authors[1].Email)
 	assert.EqualValues(t, 3, code.Authors[1].Commits)
 	assert.EqualValues(t, 5, code.Authors[0].Commits)
+
+	authors, err := GetCodeActivityAuthors(t.Context(), bareRepo1, timeFrom)
+	assert.NoError(t, err)
+	assert.Equal(t, code.Authors, authors)
 }