debugcommands: add an r prefix to make file mode for fdopen a sysstr

# skip-blame just an r prefix

Differential Revision: https://phab.mercurial-scm.org/D2562

mercurial/debugcommands.py

@@ -2257,7 +2257,7 b' def debugserve(ui, repo, **opts):'
 
     if opts['logiofd']:
         # Line buffered because output is line based.
-        logfh = os.fdopen(int(opts['logiofd']), 'ab', 1)
+        logfh = os.fdopen(int(opts['logiofd']), r'ab', 1)
     elif opts['logiofile']:
         logfh = open(opts['logiofile'], 'ab', 1)