procutil: do not convert return code of signal exit to positive number (API)

The docstring states that "codes from kill are negative", and it doesn't
make sense to make exit/signal code ambiguous.

.. api::

   ``hook.hook()`` and ``hook.runhooks()`` may return a negative integer
   to denote that the process was killed by signal.

mercurial/utils/procutil.py

@@ -83,7 +83,7 b' def explainexit(code):'
     (codes from kill are negative - not os.system/wait encoding)"""
     if code >= 0:
         return _("exited with status %d") % code, code
-    return _("killed by signal %d") % -code, -code
+    return _("killed by signal %d") % -code, code
 
 class _pfile(object):
     """File-like wrapper for a stream opened by subprocess.Popen()"""