on systems available to me, sys.exit(256) yields a zero (successful) process
exit status from the python interpreter.

certain hg subcommands return a count as their exit status (notably update,
which returns the number of unresolved files), and this gets passed directly to
sys.exit().

the sys.exit() documentation I've found:  http://docs.python.org/lib/module-sys.html
says: "Most systems require it to be in the range 0-127, and produce undefined
results otherwise."

seems like the sensible thing to do would be to clip a numeric value passed to
sys.exit to 127, either in cmdutil.runcatch(), commands.run(), or somewhere else.