lfs: override walk() in lfsvfs

In order to fix the missing lfs store after an upgrade, I attempted to walk the
store vfs to hardlink to the upgraded repo's store.  But the custom join()
clashes with the default walk() implementation.  First, 'path=None' blew up in
the regex matcher, because it wanted a string.  But even if that is fixed, the
join to walk the root of the vfs wouldn't match the required xx/xx...xx pattern.

The first cut of this was a copy/paste/tweak of the base implementation, but
this version of walk() hides the internal directories, and treats the vfs as a
flat store.  I think this makes sense because most vfs methods call join() on
input paths, which wants the simple oid format.  It also relieves the caller
from having to deal with bogus files/directories in the store.

hgext/lfs/blobstore.py

@@ -15,6 +15,7 b' from mercurial.i18n import _'
 
 from mercurial import (
     error,
+    pathutil,
     url as urlmod,
     util,
     vfs as vfsmod,
@@ -32,6 +33,28 b' class lfsvfs(vfsmod.vfs):'
             raise error.ProgrammingError('unexpected lfs path: %s' % path)
         return super(lfsvfs, self).join(path[0:2], path[2:])
 
+    def walk(self, path=None, onerror=None):
+        """Yield (dirpath, '', oids) tuple for blobs under path
+
+        Oids only exist in the root of this vfs, so dirpath is always ''.
+        """
+        root = os.path.normpath(self.base)
+        # when dirpath == root, dirpath[prefixlen:] becomes empty
+        # because len(dirpath) < prefixlen.
+        prefixlen = len(pathutil.normasprefix(root))
+        oids = []
+
+        for dirpath, dirs, files in os.walk(self.reljoin(self.base, path or ''),
+                                            onerror=onerror):
+            dirpath = dirpath[prefixlen:]
+
+            # Silently skip unexpected files and directories
+            if len(dirpath) == 2:
+                oids.extend([dirpath + f for f in files
+                             if _lfsre.match(dirpath + f)])
+
+        yield ('', '', oids)
+
 class filewithprogress(object):
     """a file-like object that supports __len__ and read.