fileset: remove callexisting flag and mctx.existing() (API) They are no longer needed since any files are included as long as they are passed in to the matcher.

fileset: rewrite predicates to return matcher not closed to subset (API) (BC) This makes fileset expression open to any input, so that we can just say "hg status 'set: not binary()'" to select text files including unknowns. With this and removal of subset computation, 'set:**' becomes as fast as 'glob:**'. Further optimization will probably be possible by narrowing the file tree to compute status for example. This also fixes 'subrepo()' to not ignore the current mctx.subset. .. bc:: The fileset expression may include untracked files by default. Use ``tracked()`` to explicitly filter out files not existing at the context revision.