verify: avoid spurious integrity warnings in verbose mode (issue6172)

The issue seems to revolve around renames in filtered commits, and only occurred
in verbose mode.  The problem occurs in the `# check renames` stage, around line
577.  Without using the unfiltered repo, this test would have printed:

    $ hg verify -v
    repository uses revlog format 1
    checking changesets
    checking manifests
    crosschecking files in changesets and manifests
    checking files
     foo@25: checking rename of 71ec0570c325: filtered revision '25'
     foobar@26: checking rename of 1b549296015b: filtered revision '26'
    checked 28 changesets with 16 changes to 11 files
    2 integrity errors encountered!
    (first damaged changeset appears to be 25)
    [1]

Differential Revision: https://phab.mercurial-scm.org/D7950

mercurial/verify.py

@@ -54,7 +54,7 b' class verifier(object):'
         self.havecl = len(repo.changelog) > 0
         self.havemf = len(repo.manifestlog.getstorage(b'')) > 0
         self.revlogv1 = repo.changelog.version != revlog.REVLOGV0
-        self.lrugetctx = util.lrucachefunc(repo.__getitem__)
+        self.lrugetctx = util.lrucachefunc(repo.unfiltered().__getitem__)
         self.refersmf = False
         self.fncachewarned = False
         # developer config: verify.skipflags

tests/test-unamend.t

@@ -346,6 +346,14 b' Testing whether unamend retains copies o'
   $ hg mv c wat
   $ hg unamend
 
+  $ hg verify -v
+  repository uses revlog format 1
+  checking changesets
+  checking manifests
+  crosschecking files in changesets and manifests
+  checking files
+  checked 28 changesets with 16 changes to 11 files
+
 Retained copies in new prdecessor commit
 
   $ hg exp --git