Mercurial 4.1 release

/!\ This is a draft and may not accurately reflect what lands in the 4.1 release.

1. Features

* A new statistical profiler has been added and made the default profiler for --profile. This profiler gives more accurate results than Python's built-in cProfile (the previous default) because the profiler overhead is much smaller. The output from --profile now displays the code hot path by default.

2. Improvements

* Performance of reading individual revlog entries has been greatly improved. This will be noticeable when performing changelog scans on large repositories (such as when executing certain revsets). * The low-level content diffing algorithm is now ~2x faster. Operations that write data to the repository (like commit) can be noticeably faster as a result.