progress: demonstrate non-linear progress has a bad ETA experience Previously, the ETA and speed assumes the progress is linear. Often, due to network or other issues, it could be fast for the most time, and suddenly slow down: [====================================================> ] \___________________________________________/\______/ very fast suddenly much slower This patch adds a test demonstrating the ETA could be way off in those cases. Differential Revision: https://phab.mercurial-scm.org/D819