Make clone.py script support checkout by date

The goal is to be able to checkout source on a branch at a specific date without creating branches

See https://stackoverflow.com/questions/6990484/git-checkout-by-date, there is 2 options one on rev-parse (limited to 90days) one on rev-list.

This is useful to pinpoint a regression without having to creates lots of branch as with:

. scripts/gitfunctions.sh 
gitf -r checkout master@{2017-04-26}
gitf -r checkout -b bench-master-20170426
gitf -r push -u origin bench-master-20170426