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.

AGENTS.md

@@ -12,6 +12,7 @@
 - In `options/locale`, only edit `locale_en-US.json`, other locales are synced automatically
 - In TS, use `!` instead of `?.`/`??` when a value always exists
 - In Go, prefer to use modern language features wherever possible
+- In Go, function-name prefixes in errors like `fmt.Errorf("Foo: %w", err)` must always name the function they are in
 - Write sizes as multiplications like `64 * 1024`, not bit shifts like `64 << 10`
 - Prefer `tw-*` utilities over inline `style` and `flex-*` helpers over per-child `tw-ml-*`/`tw-mr-*` margins, falling back to `tw-*` where specificity requires `!important`
 - Run `make fmt` after `.go` edits, `make tidy` after `go.mod` edits, `make generate-swagger` after API changes, and lint what changed with `make lint-go`, `lint-js`, `lint-css` or `lint-templates`