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