fsmonitor: do not nuke dirstate filecache In the future, chg may prefill repo's dirstate filecache so it's valuable and should be kept. Previously we drop both filecache and property cache for dirstate during fsmonitor reposetup, this patch changes it to only drop property cache but keep the filecache.

perf: move gettimer() call This is more consistent with other perf* functions.

perf: don't clobber startrev variable Previously, the "startrev" argument would be ignored due to "startrev = 0" in the benchmark function. This meant that `hg perfrevlog` always started at revision 0. Rename the local variable to "beginrev" so the variable does the right thing.