py3: fix broken man page generation, it was generating `(default: NUL*)`

`bytes(default)` was producing things like `(default: \x00)` when handed
non-bytes values such as `1`, `10`, or `True`. The man page generation would
apparently ignore these bytes and produce man pages that had the string
`(default: )`.

Test Plan:
- Ran `cd doc; python3 gendoc.py "hg.1.gendoc"` and grepped for bad output
- Ran `make deb`, extracted the deb, manually inspected `hg.1` file.

Differential Revision: https://phab.mercurial-scm.org/D8639

doc/gendoc.py

@@ -40,6 +40,7 b' from mercurial.i18n import ('
     gettext,
     _,
 )
+from mercurial.utils import stringutil
 
 table = commands.table
 globalopts = commands.globalopts
@@ -85,7 +86,9 b' def get_opts(opts):'
         if b'\n' in desc:
             # only remove line breaks and indentation
             desc = b' '.join(l.lstrip() for l in desc.split(b'\n'))
-        desc += default and _(b" (default: %s)") % bytes(default) or b""
+        if default:
+            default = stringutil.forcebytestr(default)
+            desc += _(b" (default: %s)") % default
         yield (b", ".join(allopts), desc)