debugcommands: force import of fileset in debugfileset

It looks like Python 3's lazy importer is better than Python 2's for
this command, and as a result we had no symbols in the filesetlang
symbol table, which resulted in some really mysterious test-fileset.t
failures around withstatus optimizations. Inserting this explicit
import and forcing its evaluation fixes the test failure.

Differential Revision: https://phab.mercurial-scm.org/D4172

mercurial/debugcommands.py

@@ -896,6 +896,8 b' def debugextensions(ui, repo, **opts):'
     _('[-r REV] [--all-files] [OPTION]... FILESPEC'))
 def debugfileset(ui, repo, expr, **opts):
     '''parse and apply a fileset specification'''
+    from . import fileset
+    fileset.symbols # force import of fileset so we have predicates to optimize
     opts = pycompat.byteskwargs(opts)
     ctx = scmutil.revsingle(repo, opts.get('rev'), None)