localrepo: extract loading of hgrc files to standalone function Various 3rd party extensions supplement where per-repo config data lives. Looking at their sources, they resort to unorthodox means to inject the config data. And the way they do it is susceptible to corner cases. e.g. not processing automatic extension loads, not reacting to new or disabled extensions in configs, etc. This commit extracts the core logic of loading hgrc files into a standalone function so there is a clear function that can be monkeypatched to inject per-repo config data at repository open time. Differential Revision: https://phab.mercurial-scm.org/D5221