procutil: avoid `+= None` when writing to full std{err,out} descriptor on py3

The write function returns `None` if there was no room to write the given
data[1].  I don't like that this is effectively an infinite loop if there's
never any progress emptying the underlying buffer, but we're no worse off than
before, and it fixes random stacktrace popups seen in the py3 build of
TortoiseHg.

[1] https://docs.python.org/3/library/io.html#io.RawIOBase.write

Differential Revision: https://phab.mercurial-scm.org/D12555

mercurial/utils/procutil.py

@@ -112,7 +112,9 b' class WriteAllWrapper(object):'
         total_to_write = len(s)
         total_written = 0
         while total_written < total_to_write:
-            total_written += write1(m[total_written:])
+            c = write1(m[total_written:])
+            if c:
+                total_written += c
         return total_written