add the ability to specify repository ref (#80)

closes #75

Reviewed-on: #80
Co-authored-by: Maxim Slipenko <no-reply@maxim.slipenko.com>
Co-committed-by: Maxim Slipenko <no-reply@maxim.slipenko.com>

pkg/repos/utils.go

@@ -18,12 +18,18 @@ package repos
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"path/filepath"
 	"reflect"
 	"strings"
 
+	"github.com/go-git/go-git/v5"
+	"github.com/go-git/go-git/v5/plumbing"
 	"github.com/go-git/go-git/v5/plumbing/format/diff"
+	"github.com/go-git/go-git/v5/plumbing/transport"
+	"github.com/go-git/go-git/v5/plumbing/transport/client"
+
 	"mvdan.cc/sh/v3/interp"
 	"mvdan.cc/sh/v3/syntax"
 
@@ -137,3 +143,59 @@ func resolveOverrides(runner *interp.Runner, pkg *db.Package) {
 		}
 	}
 }
+
+func getHeadReference(r *git.Repository) (plumbing.ReferenceName, error) {
+	remote, err := r.Remote(git.DefaultRemoteName)
+	if err != nil {
+		return "", err
+	}
+
+	endpoint, err := transport.NewEndpoint(remote.Config().URLs[0])
+	if err != nil {
+		return "", err
+	}
+
+	gitClient, err := client.NewClient(endpoint)
+	if err != nil {
+		return "", err
+	}
+
+	session, err := gitClient.NewUploadPackSession(endpoint, nil)
+	if err != nil {
+		return "", err
+	}
+
+	info, err := session.AdvertisedReferences()
+	if err != nil {
+		return "", err
+	}
+
+	refs, err := info.AllReferences()
+	if err != nil {
+		return "", err
+	}
+
+	return refs["HEAD"].Target(), nil
+}
+
+func resolveHash(r *git.Repository, ref string) (*plumbing.Hash, error) {
+	var err error
+
+	if ref == "" {
+		reference, err := getHeadReference(r)
+		if err != nil {
+			return nil, fmt.Errorf("failed to get head reference %w", err)
+		}
+		ref = reference.Short()
+	}
+
+	hsh, err := r.ResolveRevision(git.DefaultRemoteName + "/" + plumbing.Revision(ref))
+	if err != nil {
+		hsh, err = r.ResolveRevision(plumbing.Revision(ref))
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return hsh, nil
+}