Release 13.1

This update to Release 13 fixes errors that occur from the new default branch name checking code when used with versions of git that are too old. This code is now disabled if the git version is not capable of running it, and in verbose mode a warning is printed. The warning can be disabled by explicitly setting "skip_git_default_check => 1" in the config file. In either case, the owner will need to update their git installation or remove all branch and mirror repositories when the default branch name changes.