hgweb: profile HTTP requests Currently, running `hg serve --profile` doesn't yield anything useful: when the process is terminated the profiling output displays results from the main thread, which typically spends most of its time in select.select(). Furthermore, it has no meaningful results from mercurial.* modules because the threads serving HTTP requests don't actually get profiled. This patch teaches the hgweb wsgi applications to profile individual requests. If profiling is enabled, the profiler kicks in after HTTP/WSGI environment processing but before Mercurial's main request processing. The profile results are printed to the configured profiling output. If running `hg serve` from a shell, they will be printed to stderr, just before the HTTP request line is logged. If profiling to a file, we only write a single profile to the file because the file is not opened in append mode. We could add support for appending to files in a future patch if someone wants it. Per request profiling doesn't work with the statprof profiler because internally that profiler collects samples from the thread that *initially* requested profiling be enabled. I have plans to address this by vendoring Facebook's customized statprof and then improving it.