fix(changelog): command does not fail when there was no commit since …

…last tag

Release failed with a cryptic git error, when there was no commit since last
tag. Now command exits with `no commits since last tag` error message.

Closes: #1

internal/cmd/release.go

@@ -42,6 +42,10 @@ func (c Command) runRelease(dst io.Writer, l *flash.Logger, cfg config.Changelog
 		return err
 	}
 
+	if len(revs) == 0 {
+		return errors.New("no commits since last tag")
+	}
+
 	cw, err := c.createChangelog(g, cfg, l, revs)
 	if err != nil {
 		return err

internal/git/git.go

@@ -109,12 +109,18 @@ func (c Command) RevList(start, end string) ([]string, error) {
 		arg = start + ".." + end
 	}
 
-	refs, err := c.Run("rev-list", arg)
+	revs, err := c.Run("rev-list", arg)
 	if err != nil {
 		return nil, err
 	}
 
-	return strings.Split(strings.TrimSpace(refs), "\n"), nil
+	revs = strings.TrimSpace(revs)
+
+	if revs == "" {
+		return []string{}, nil
+	}
+
+	return strings.Split(strings.TrimSpace(revs), "\n"), nil
 }
 
 // CreateRelease creates a release tag.