hgweb: abstract call to hgwebdir wsgi function The function names and behavior now matches hgweb. The reason for this will be obvious in the next patch.

profiling: don't error with statprof when profiling has already started statprof.reset() asserts if profiling has already started. So don't call if it profiling is already running.

profiling: add a context manager that no-ops if profiling isn't enabled And refactor dispatch.py to use it. As you can see, the resulting code is much simpler. I was tempted to inline _runcommand as part of writing this series. However, a number of extensions wrap _runcommand. So keeping it around is necessary (extensions can't easily wrap runcommand because it calls hooks before and after command execution).