storageutil: extract copy metadata retrieval out of filelog

As part of implementing an alternate storage backend, I found myself
reinventing this wheel.

Let's create a utility function for doing the work.

Differential Revision: https://phab.mercurial-scm.org/D4800

mercurial/filelog.py

@@ -115,15 +115,7 class filelog(object):
         return self.addrevision(text, transaction, link, p1, p2)
 
     def renamed(self, node):
-        if self.parents(node)[0] != revlog.nullid:
-            return False
-        t = self.revision(node)
-        m = storageutil.parsemeta(t)[0]
-        # copy and copyrev occur in pairs. In rare cases due to bugs,
-        # one can occur without the other.
-        if m and "copy" in m and "copyrev" in m:
-            return (m["copy"], revlog.bin(m["copyrev"]))
-        return False
+        return storageutil.filerevisioncopied(self, node)
 
     def size(self, rev):
         """return the size of a given revision"""

mercurial/utils/storageutil.py

@@ -89,6 +89,25 def filtermetadata(text):
     offset = text.index(b'\x01\n', 2)
     return text[offset + 2:]
 
+def filerevisioncopied(store, node):
+    """Resolve file revision copy metadata.
+
+    Returns ``False`` if the file has no copy metadata. Otherwise a
+    2-tuple of the source filename and node.
+    """
+    if store.parents(node)[0] != nullid:
+        return False
+
+    meta = parsemeta(store.revision(node))[0]
+
+    # copy and copyrev occur in pairs. In rare cases due to old bugs,
+    # one can occur without the other. So ensure both are present to flag
+    # as a copy.
+    if meta and b'copy' in meta and b'copyrev' in meta:
+        return meta[b'copy'], bin(meta[b'copyrev'])
+
+    return False
+
 def iterrevs(storelen, start=0, stop=None):
     """Iterate over revision numbers in a store."""
     step = 1