revlog: split the content verification of a node into a separate method

This will be used by LFS to tune what is skipped.

In the future, this could also be used by LFS to indicate which nodes tagged
with `skipread` are simply in need of a blob fetch, so that they can be done in
a batch later.  (Currently, `skipread` also indicates censored data and errors.)
Additionally, it could be used to cache the sha1 hash value for each blob so
that large blobs don't need to be re-read and re-hashed if they are used by
multiple nodes.

Differential Revision: https://phab.mercurial-scm.org/D7710

mercurial/revlog.py

@@ -148,6 +148,16 b' def offset_type(offset, type):'
     return int(int(offset) << 16 | type)
 
 
+def _verify_revision(rl, skipflags, state, node):
+    """Verify the integrity of the given revlog ``node`` while providing a hook
+    point for extensions to influence the operation."""
+    if skipflags:
+        state[b'skipread'].add(node)
+    else:
+        # Side-effect: read content and verify hash.
+        rl.revision(node)
+
+
 @attr.s(slots=True, frozen=True)
 class _revisioninfo(object):
     """Information about a revision that allows building its fulltext
@@ -2914,11 +2924,7 b' class revlog(object):'
                 if skipflags:
                     skipflags &= self.flags(rev)
 
-                if skipflags:
-                    state[b'skipread'].add(node)
-                else:
-                    # Side-effect: read content and verify hash.
-                    self.revision(node)
+                _verify_revision(self, skipflags, state, node)
 
                 l1 = self.rawsize(rev)
                 l2 = len(self.rawdata(node))