filelog: store filename directly on revlog instance This attribute is only used by LFS. It is used by one of the revlog flag processor functions, which gets an instance of the revlog - not the file storage type. So, it makes sense to store this attribute on the revlog instead of the filelog. With this change, I'm pretty confident that LFS is no longer directly accessing file storage interface members that are revlog centric. i.e. it gets us one step closer to eliminating revlog-centric APIs from the file storage interface! Differential Revision: https://phab.mercurial-scm.org/D4715