perftemplating: move revision argument to flag only The revision we want to render are less important than the templates. We move revision specification behind the usual `--rev` flag.

perftemplating: drop usage of buffer The buffer can consume a lot of memory and change various internal behaviors. Writing to dev/null seems more appropriate when it comes to benchmark.

perftemplating: move template formating into its own function The lambda is not practical when time will come to alter the benchmark code.

perftemplating: fix the revisions argument Before this change, the argument passed to log where a string instead of a list of string. This meant only single character rev were supported... We now properly accepts argument of any length. We also make it possible to specify multiple REV arguments in the same go.