Performance improvement for last commit cache and show-ref (#15455)

* Improve performance when there are multiple commits in the last commit cache

* read refs directly if we can

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/repo_commit_nogogit.go

@@ -9,9 +9,10 @@ package git
 import (
 	"bufio"
 	"errors"
-	"fmt"
 	"io"
 	"io/ioutil"
+	"os"
+	"path/filepath"
 	"strings"
 )
 
@@ -34,6 +35,18 @@ func (repo *Repository) ResolveReference(name string) (string, error) {
 
 // GetRefCommitID returns the last commit ID string of given reference (branch or tag).
 func (repo *Repository) GetRefCommitID(name string) (string, error) {
+	if strings.HasPrefix(name, "refs/") {
+		// We're gonna try just reading the ref file as this is likely to be quicker than other options
+		fileInfo, err := os.Lstat(filepath.Join(repo.Path, name))
+		if err == nil && fileInfo.Mode().IsRegular() && fileInfo.Size() == 41 {
+			ref, err := ioutil.ReadFile(filepath.Join(repo.Path, name))
+
+			if err == nil && SHAPattern.Match(ref[:40]) && ref[40] == '\n' {
+				return string(ref[:40]), nil
+			}
+		}
+	}
+
 	stdout, err := NewCommand("show-ref", "--verify", "--hash", name).RunInDir(repo.Path)
 	if err != nil {
 		if strings.Contains(err.Error(), "not a valid ref") {
@@ -69,6 +82,11 @@ func (repo *Repository) getCommit(id SHA1) (*Commit, error) {
 	}()
 
 	bufReader := bufio.NewReader(stdoutReader)
+
+	return repo.getCommitFromBatchReader(bufReader, id)
+}
+
+func (repo *Repository) getCommitFromBatchReader(bufReader *bufio.Reader, id SHA1) (*Commit, error) {
 	_, typ, size, err := ReadBatchLine(bufReader)
 	if err != nil {
 		if errors.Is(err, io.EOF) {
@@ -106,7 +124,6 @@ func (repo *Repository) getCommit(id SHA1) (*Commit, error) {
 	case "commit":
 		return CommitFromReader(repo, id, io.LimitReader(bufReader, size))
 	default:
-		_ = stdoutReader.CloseWithError(fmt.Errorf("unknown typ: %s", typ))
 		log("Unknown typ: %s", typ)
 		return nil, ErrNotExist{
 			ID: id.String(),