revsetbenchmarks: use combination variants in default set Now that we have them, let's make use of them.

revsetbenchmarks: support combining variants with "+" We need more advanced variants in some cases. For example, "The last rev of the sorted version". We introduce a syntax for this: `reverse+last` means `last(reverse(REVSET))`.

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.

revsetbenchmarks: display even more compact timing result We now use an 8 char display for timing (from 10), we add some logic to drop precision if the number grows too large (as we do not care about sub-0 digit in this case). This allow to pack more variants in a single screen.