Avoid creating an array for the sole purpose of counting elements (#17)

* Avoid creating an array for the sole purpose of counting elements

Probably speeds up counting commits for git versions < 1.8.0,
although I dubt it would make a visible difference

* Fix commit count with git version < 1.8.0

With format='' the output does not end with a newline (checked)

commit.go

@@ -158,7 +158,7 @@ func commitsCount(repoPath, revision, relpath string) (int64, error) {
 	}
 
 	if isFallback {
-		return int64(len(strings.Split(stdout, "\n"))), nil
+		return int64(strings.Count(stdout, "\n")) + 1, nil
 	}
 	return strconv.ParseInt(strings.TrimSpace(stdout), 10, 64)
 }