match: make 'match.files()' return list object always

'exact' match objects are sometimes created with a non-list 'pattern'
argument:

  - using 'set' in queue.refresh():hgext/mq.py
        match = scmutil.matchfiles(repo, set(c[0] + c[1] + c[2] + inclsubs))

  - using 'dict' in revert():mercurial/cmdutil.py (names = {})
        m = scmutil.matchfiles(repo, names)

'exact' match objects return specified 'pattern' to callers of
'match.files()' as it is, so it is a non-list object.

but almost all implementations expect 'match.files()' to return a list
object, so this may causes problems: e.g. exception for "+" with
another list object.

this patch ensures that '_files' of 'exact' match objects is a list
object.

for non 'exact' match objects, parsing specified 'pattern' already
ensures that it it a list one.

mercurial/match.py

@@ -62,7 +62,10 b' class match(object):'
             pats = _normalize(exclude, 'glob', root, cwd, auditor)
             self.excludepat, em = _buildmatch(ctx, pats, '(?:/|$)')
         if exact:
-            self._files = patterns
+            if isinstance(patterns, list):
+                self._files = patterns
+            else:
+                self._files = list(patterns)
             pm = self.exact
         elif patterns:
             pats = _normalize(patterns, default, root, cwd, auditor)