py3: pass str into RuntimeError() to prevent b'' in output

# skip-blame as just r'' prefixes

Differential Revision: https://phab.mercurial-scm.org/D5740

hgext/remotefilelog/shallowutil.py

@@ -237,9 +237,9 b' def parsesizeflags(raw):'
             # v0, str(int(size)) is the header
             size = int(header)
     except ValueError:
-        raise RuntimeError("unexpected remotefilelog header: illegal format")
+        raise RuntimeError(r"unexpected remotefilelog header: illegal format")
     if size is None:
-        raise RuntimeError("unexpected remotefilelog header: no size found")
+        raise RuntimeError(r"unexpected remotefilelog header: no size found")
     return index + 1, size, flags
 
 def buildfileblobheader(size, flags, version=None):