evolve: fix progress display with hg<4.7
Since d1aab9d82f5b (evolve: adapt for deprecated ui.progress(),
2019-01-14), the "item", "units" and "total" are all set to default
values. The last one is especially bad since it results in a "spinner"
instead of a progress bar.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate -I 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null