verify: add a hook that can let extensions manipulate file lists

Without a hook of this nature, narrowhg[0] clones always result in 'hg
verify' reporting terrible damage to the entire repository
history. With this hook, we can ignore files that aren't supposed to
be in the clone, and then get an accurate report of any damage present
(or not) in the repo.

0: https://bitbucket.org/Google/narrowhg

mercurial/verify.py

@@ -35,6 +35,17 b' def _normpath(f):'
         f = f.replace('//', '/')
     return f
 
+def _validpath(repo, path):
+    """Returns False if a path should NOT be treated as part of a repo.
+
+    For all in-core cases, this returns True, as we have no way for a
+    path to be mentioned in the history but not actually be
+    relevant. For narrow clones, this is important because many
+    filelogs will be missing, and changelog entries may mention
+    modified files that are outside the narrow scope.
+    """
+    return True
+
 def _verify(repo):
     repo = repo.unfiltered()
     mflinkrevs = {}
@@ -154,7 +165,8 b' def _verify(repo):'
                 mflinkrevs.setdefault(changes[0], []).append(i)
                 refersmf = True
             for f in changes[3]:
-                filelinkrevs.setdefault(_normpath(f), []).append(i)
+                if _validpath(repo, f):
+                    filelinkrevs.setdefault(_normpath(f), []).append(i)
         except Exception as inst:
             refersmf = True
             exc(i, _("unpacking changeset %s") % short(n), inst)
@@ -181,7 +193,9 b' def _verify(repo):'
                 if not f:
                     err(lr, _("file without name in manifest"))
                 elif f != "/dev/null": # ignore this in very old repos
-                    filenodes.setdefault(_normpath(f), {}).setdefault(fn, lr)
+                    if _validpath(repo, f):
+                        filenodes.setdefault(
+                            _normpath(f), {}).setdefault(fn, lr)
         except Exception as inst:
             exc(lr, _("reading manifest delta %s") % short(n), inst)
     ui.progress(_('checking'), None)