branchmap: force Exception to bytes before logging stable

branchmap: force Exception to bytes before logging

Here was an instance where `black` mangled the formatting so that `pytype`
didn't recognize the suppression directive.  But it seems that the error was
correct, and the code should follow other recent changes around exception
conversion.

    File "/mnt/c/Users/Matt/hg/mercurial/branchmap.py", line 303, in fromfile: Function _bytestr.__init__ was called with the wrong arguments [wrong-arg-types]
             Expected: (self, ints: Iterable[int])
      Actually passed: (self, ints: Exception)
      The following methods aren't implemented on Exception:
      __iter__

Differential Revision: https://phab.mercurial-scm.org/D10170

--- a/mercurial/branchmap.py	Thu Mar 11 17:05:37 2021 -0500
+++ b/mercurial/branchmap.py	Thu Mar 11 17:10:52 2021 -0500
@@ -299,9 +299,7 @@
                     msg
                     % (
                         _branchcachedesc(repo),
-                        pycompat.bytestr(
-                            inst
-                        ),  # pytype: disable=wrong-arg-types
+                        stringutil.forcebytestr(inst),
                     )
                 )
             bcache = None