merge: force an exception message to bytes before printing as a warning

Caught by pytype:

    File "/mnt/c/Users/Matt/hg/mercurial/merge.py", line 1346, in batchremove: Function _bytestr.__init__ was called with the wrong arguments [wrong-arg-types]
             Expected: (self, ints: Iterable[int])
      Actually passed: (self, ints: str)

Differential Revision: https://phab.mercurial-scm.org/D10177

mercurial/merge.py

@@ -20,6 +20,7 b' from .node import ('
     nullrev,
 )
 from .thirdparty import attr
+from .utils import stringutil
 from . import (
     copies,
     encoding,
@@ -1341,7 +1342,7 b' def batchremove(repo, wctx, actions):'
         except OSError as inst:
             repo.ui.warn(
                 _(b"update failed to remove %s: %s!\n")
-                % (f, pycompat.bytestr(inst.strerror))
+                % (f, stringutil.forcebytestr(inst.strerror))
             )
         if i == 100:
             yield i, f