match: fix assertion for fileset with no context (issue6046)

A falsy changectx should be allowed.

mercurial/match.py

@@ -48,7 +48,7 b' def _expandsets(root, cwd, kindpats, ctx'
 
     for kind, pat, source in kindpats:
         if kind == 'set':
-            if not ctx:
+            if ctx is None:
                 raise error.ProgrammingError("fileset expression with no "
                                              "context")
             matchers.append(ctx.matchfileset(pat, badfn=badfn))

tests/test-locate.t

@@ -156,6 +156,11 b' Issue294: hg remove --after dir fails wh'
   $ hg files .
   [1]
 
+Fileset at null (i.e. a falsy context) shouldn't crash (issue6046)
+
+  $ hg files -r null 'set:tracked()'
+  [1]
+
 Convert native path separator to slash (issue5572)
 
   $ hg files -T '{path|relpath|slashpath}\n'