lfs: add a local store method for opening a blob

The has() and read() methods already dynamically switch between the usercache
and local store.  This should generally be preferred to directly accessing the
vfs instances outside of the store.

The file is now explicitly opened in binary mode for clarity.  (It was also
being opened in binary mode before, but only because vfs.__call__() appends 'b'
if needed when not opening with 'text=True'.)

hgext/lfs/blobstore.py

@@ -100,6 +100,14 b' class local(object):'
         self.cachevfs = lfsvfs(usercache)
         self.ui = repo.ui
 
+    def open(self, oid):
+        """Open a read-only file descriptor to the named blob, in either the
+        usercache or the local store."""
+        if self.cachevfs.exists(oid):
+            return self.cachevfs(oid, 'rb')
+
+        return self.vfs(oid, 'rb')
+
     def write(self, oid, data, verify=True):
         """Write blob to local blobstore."""
         if verify: