bookmarks: use correct store for "ambiguity check"

I still don't quite know what the check does, but I clearly got it
wrong in 526750cdd02d (bookmarks: keep bookmarks in .hg/store if new
config set, 2019-05-15). Just compare with the strings we use in
@repofilecache and @storecache. These bugs were then copied to the
stable branch in c2b83c957621 (localrepo: grab mixedrepostorecache
class from 526750cdd02d, 2019-05-20) and 2338bdea4474 (bookmark: also
make bookmark cache depends of the changelog, 2019-05-20). As a
result, test-wireproto-exchangev2.t is flaky on both branches. This
patch fixes that.

Differential Revision: https://phab.mercurial-scm.org/D6469

mercurial/localrepo.py

@@ -133,10 +133,10 b' class mixedrepostorecache(_basefilecache'
 
     def join(self, obj, fnameandlocation):
         fname, location = fnameandlocation
-        if location == '':
+        if location == 'plain':
             return obj.vfs.join(fname)
         else:
-            if location != 'store':
+            if location != '':
                 raise error.ProgrammingError('unexpected location: %s' %
                                              location)
             return obj.sjoin(fname)
@@ -1219,8 +1219,8 b' class localrepository(object):'
         cls = repoview.newtype(self.unfiltered().__class__)
         return cls(self, name, visibilityexceptions)
 
-    @mixedrepostorecache(('bookmarks', ''), ('bookmarks.current', ''),
-                         ('00changelog.i', 'store'))
+    @mixedrepostorecache(('bookmarks', 'plain'), ('bookmarks.current', 'plain'),
+                         ('00changelog.i', ''))
     def _bookmarks(self):
         return bookmarks.bmstore(self)