allow two-step benchmarks with Porting/bench.pl

allow two-step benchmarks with Porting/bench.pl

with this patch you can time blead, and save the results:

    bench.pl --tests=/whatever/ --write=blead.time -- ./perl=blead

then iterate over hacking the code and running

    bench.pl --read=blead.time --write=last.time -- ./perl=hacked

and seeing the results of the --tests defined in the first run,
and the results from the save file 'blead.time' compared against
the modified version of perl in the most recent run. At the same
time the merged data is written as last.time, allowing the usual
post-test analysis of the results.