perf: make hg perfwrite more flexible
The more flexible command was used recently while finding a solution for a
buffering bug (eventually fixed in f9734b2d59cc (the changeset description uses
a different benchmark)).
In comparison to the previous version, the new version is much more flexible.
While using it, the focus was on testing small writes. For this reason, by
default it calls ui.write() 100 times with a single byte plus one newline byte,
for 100 lines.
To get the previous behavior, run `hg perfwrite --nlines=100000 --nitems=1
--item='Testing write performance' --batch-line`.