perf: stop creating new revlog by hand in perftags It's better to let the repository logic create its own object. We now just clear the cache. New object will be automatically created from there.

revlog: update the documentation for `trim_endidx` The function role drifted since the function was commented.

revlog: properly detect corrupted revlog in `index_get_length` Pointed out by Yuya Nishihara.

perf: rename `perfhelper-tracecopies` to `perfhelper-pathcopies` The command it supports is called `perfpathcopies`. It seems better to align the names.