debugcommands: fix a missing b prefix

This wasn't causing any immediate problems because of the source
transformer, but I noticed the inconsistency and it bugged me.

# skip-blame just a b prefix

Differential Revision: https://phab.mercurial-scm.org/D4255

mercurial/debugcommands.py

@@ -2721,7 +2721,7 b' def _parsewirelangblocks(fh):'
         if line.startswith(b'#'):
             continue
 
-        if not line.startswith(' '):
+        if not line.startswith(b' '):
             # New block. Flush previous one.
             if activeaction:
                 yield activeaction, blocklines