localrepo: document and test bug around opening shared repos As part of refactoring this code, I realized that we don't validate the requirements of a shared repository. This commit documents that next to the requirements validation code and adds a test demonstrating the buggy behavior. I'm not sure if I'll fix this. But it is definitely a bug that users could encounter, as LFS, narrow, and potentially other extensions dynamically add requirements on first use. One part of this I'm not sure about is how to handle loading the .hg/hgrc of the shared repo. We need to do that in order to load extensions. But we don't want that repo's hgrc to overwrite the current repo's. Differential Revision: https://phab.mercurial-scm.org/D4572