memctx: allow extensions to determine what filectxfn should do Rev 650b5b6e75ed switched the contract for filectxfn from "raise IOError if file is missing" to "return None if file is missing". Out of tree extensions need to be updated for that, but for extensions interested in compatibility with both Mercurial <= 3.1 and default, it is next to impossible to introspect core Mercurial to figure out what to do. This patch adds a field to memctx for extensions to use.