verify: also check full manifest validity during verify runs

Before this changes, `hg verify` only checked if a manifest revision existed and
referenced the proper files. However it never checked the manifest revision
content itself.

Mercurial is expecting manifest entries to be sorted and will crash otherwise.
Since `hg verify` did not attempted a full restoration of manifest entry, it
could ignore this kind of corruption.

This new check significantly increases the cost of a `hg verify` run. This
especially affects large repository not using `sparse-revlog`. For now, this is
hidden behind the `--full` experimental flag.

mercurial/verify.py

@@ -337,6 +337,16 b' class verifier(object):'
                         filenodes.setdefault(fullpath, {}).setdefault(fn, lr)
             except Exception as inst:
                 self._exc(lr, _("reading delta %s") % short(n), inst, label)
+            if self._level >= VERIFY_FULL:
+                try:
+                    # Various issues can affect manifest. So we read each full
+                    # text from storage. This triggers the checks from the core
+                    # code (eg: hash verification, filename are ordered, etc.)
+                    mfdelta = mfl.get(dir, n).read()
+                except Exception as inst:
+                    self._exc(lr, _("reading full manifest %s") % short(n),
+                              inst, label)
+
         if not dir:
             progress.complete()