progress: create helper class for incrementing progress When using ui.progress(), there's a clear pattern that is followed: * Pass the same topic and unit * Usually pass the same total * Call with pos=None to close the progress bar * Often keep track of the current position and increment it This patch creates a simple helper class for this. I'll probably make it implement the context manager protocol later (calling update(None) on __exit__). Progress is used in low-level modules like changegroup, so I also exposed it via a method on the ui object. Perhaps the class itself should also live in ui.py? This patch also makes merge.oy use it to show that it works. Differential Revision: https://phab.mercurial-scm.org/D3765