lfs: add a blob verification method to the local store

A corrupt blob can be signaled through the Batch API response, without actually
transferring the file.  A true/false indicator is slightly easier than
immediately catching an exception.

hgext/lfs/blobstore.py

@@ -175,6 +175,17 b' class local(object):'
             _verify(oid, blob)
         return blob
 
+    def verify(self, oid):
+        """Indicate whether or not the hash of the underlying file matches its
+        name."""
+        sha256 = hashlib.sha256()
+
+        with self.open(oid) as fp:
+            for chunk in util.filechunkiter(fp, size=1048576):
+                sha256.update(chunk)
+
+        return oid == sha256.hexdigest()
+
     def has(self, oid):
         """Returns True if the local blobstore contains the requested blob,
         False otherwise."""