evolvecmd: don't update working copy when using in-memory merge

This patch removes the update of the working copy after each in-memory
merge and only instead updates the working copy afterwards (if
requested).

There's still no significant speedup in the hg repo. It seems that the
cost of updating the working copy is not large enough to make much
difference, and the cost of reading and updating obsmarkers is
relatively high in that repo. A significant part of the time (~35%) is
spent in `repoview.computehidden()` because it is recalculated for
every commit (seems like potential for improvement).

I made similar changes in the mozilla-unified repo (which has a
significantly larger working copy), by adding 10 commits each changing
one line of a file. There, evolving 9 of those commits took 34s before
this patch and 20s after. I measured similar speedups in an internal
repo (9.0s -> 5.2s).
19 jobs for topic/default/in-memory in 4 minutes and 42 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed checks-py2 #165762

00:02:51

passed checks-py3 #165763

00:01:50

passed doc #166467

00:00:16

manual pytype #165761
allowed to fail manual
passed tests-py2-cext #165764

00:04:18

passed tests-py2-pure #165765

00:04:25

passed tests-py3-cext #165766

00:04:15

passed tests-py3-pure #165767

00:02:35

passed doc #166466

00:00:27

passed doc #165977

00:00:36

failed doc #165974

00:00:37

failed doc #165768

00:00:37

passed doc #166465

00:00:26

passed doc #166464

00:00:24

failed doc #165976

00:00:42

failed doc #165975

00:00:31

failed doc #165973

00:00:41

failed doc #165962

00:00:20

failed doc #165892

00:00:38