filelog: add a hasnode() method (API) Missing in the file storage interface is the ability to query whether a specified value is a known node. This commit defines that interface member and implements it on the revlog and sqlite file stores. Storage unit tests have been added. The revlog implementation is a bit more complicated because index lookups don't consistently raise the same exception. For SQLite, we can simply look for a key in a dict. Differential Revision: https://phab.mercurial-scm.org/D5163