revsetbenchmark: automatically finds the perf extension Before this changeset, you had to stand in the root of the mercurial repo to run the `revsetbenchmark.py` script. Otherwise, the perf extension would not be found a `./contrib/perf.py` and the script would crash in panic. We now figure out the contrib directory from the location of this script. This makes it possible to run the script from other location that the mercurial repo root (but you still need to be in the core mercurial repository)