keyword: make wrapped repository and kwtemplater refer to each other Wrapper functions of keyword extension are defined in reposetup(), because they refer to kwtemplater instantiated in reposetup(). But these functions can be defined statically, if kwtemplater can be obtained via repository at runtime. This is a part of preparations for defining them statically. To avoid cyclic reference, this patch makes kwtemplater use weakref to refer related repository instance.

keyword: add test for keyword expansion at serving multiple repositories This is safety for subsequent (and future) patches, which change function wrapping.