revsetbenchmarks: use many more variants by default

So far the variants feature was introduced, but not used by
default. We now use a set of basic variants by default.

contrib/revsetbenchmarks.py

@@ -16,7 +16,7 b' from subprocess import check_call, Popen'
 # cannot use argparse, python 2.7 only
 from optparse import OptionParser
 
-DEFAULTVARIANTS = ['plain']
+DEFAULTVARIANTS = ['plain', 'min', 'max', 'first', 'last', 'reverse', 'sort']
 
 def check_output(*args, **kwargs):
     kwargs.setdefault('stderr', PIPE)