perf: use a formatter for output We use a `formatter` object in the perf extensions. This allow the use of formatted output like json. To avoid adding logic to create a formatter and pass it around to the timer function in every command, we add a `gettimer` function in charge of returning a `timer` function as simple as before but embedding an appropriate formatter. This new `gettimer` function also return the formatter as it needs to be explicitly closed at the end of the command. example output: $ hg --config ui.formatjson=True perfvolatilesets visible obsolete [ { "comb": 0.02, "count": 126, "sys": 0.0, "title": "obsolete", "user": 0.02, "wall": 0.0199398994446 }, { "comb": 0.02, "count": 117, "sys": 0.0, "title": "visible", "user": 0.02, "wall": 0.0250301361084 } ]