simplestore: use a custom store for the simple store repo Before, we used the default store, which was based on fncache and dotencode. After attempting to port tests to work with the simple store, I realized that fncache was more trouble than it is worth. This commit implements a proper store type for the simple repo - one that isn't based off fncache. This causes a number of new test failures because of tests expecting the full fncache store filename encoding. I may extend the store format in a subsequent commit to take the filename encoding parts of fncache that we can take (basically everything except hash encoding, since that isn't reversible). But for now, let's use encoded store. As part of this, we implement proper requirements support for repos created with the simple store. This should have been done from the beginning, as a requirement is needed to lock out clients that don't understand a storage format. A new hghave feature advertising the presence of fncache in repos has been added. Most tests touching the fncache are now conditional on that feature. Other tests have added the optional repo requirement to output. Differential Revision: https://phab.mercurial-scm.org/D3095