Fix --same-loops flag in compile subprocess invocation (#475)

When the compile subcommand spawns a "pyperformance run"
subprocess, it constructs --same_loops on the command line
where argparse only accepts --same-loops. Change the flag to
use the hyphenated form.

Author: stratakis

pyperformance/compile.py

@@ -571,7 +571,7 @@ def run_benchmark(self, python=None):
         if self.conf.debug:
             cmd.append("--debug-single-value")
         if self.conf.same_loops:
-            cmd.append("--same_loops=%s" % self.conf.same_loops)
+            cmd.append("--same-loops=%s" % self.conf.same_loops)
         if self.conf.rigorous:
             cmd.append("--rigorous")
         exitcode = self.run_nocheck(*cmd)