Fetch the most expensive info in parallel

Operations that need to walk the history are run in parallel, in
particular the last-commit-per-file info is obtained in parallel.  This
cuts down our response rate drastically, although it's still way too
high.

quit.go

@@ -13,6 +13,7 @@ import (
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"time"
	"github.com/libgit2/git2go"
@@ -42,11 +43,58 @@ func main() {
	http.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
		performance.MeasureRequest(req)
		handleError := func(w http.ResponseWriter, err error) {
			log.Println("git: ", err)
			http.Error(w, "internal server error", http.StatusInternalServerError)
		}
		fancyRepo := NewFancyRepo(repo)
		commit, err := fancyRepo.Commit()
		if err != nil {
			handleError(w, err)
			return
		}
		files, err := commit.Files(false)
		if err != nil {
			handleError(w, err)
			return
		}
		var wg sync.WaitGroup
		wg.Add(len(files))
		for _, f := range files {
			go func(f *FancyFile) {
				defer wg.Done()
				_, err := f.Commit()
				if err != nil {
					handleError(w, err)
				}
			}(f)
		}
		wg.Add(2)
		go func() {
			defer wg.Done()
			_, err := fancyRepo.CommitCount()
			if err != nil {
				handleError(w, err)
			}
		}()
		go func() {
			defer wg.Done()
			_, err := fancyRepo.Contributors()
			if err != nil {
				handleError(w, err)
			}
		}()
		wg.Wait()
		buf := new(bytes.Buffer)
		start := time.Now()
		err = repoTmpl.Execute(buf, map[string]interface{}{
			"RepoPath":    path.Base(repoPath),
			"Repo":        NewFancyRepo(repo),
			"Repo":        fancyRepo,
			"Style":       template.CSS(repoStyle),
			"Performance": performance.Render(req),
		})