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 #165762
checks-py2

00:02:51

passed #165763
checks-py3

00:01:50

passed #166467
doc

00:00:16

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

00:04:18

passed #165765
tests-py2-pure

00:04:25

passed #165766
tests-py3-cext

00:04:15

passed #165767
tests-py3-pure

00:02:35

passed #166466
doc

00:00:27

passed #166465
doc

00:00:26

passed #166464
doc

00:00:24

passed #165977
doc

00:00:36

failed #165976
doc

00:00:42

failed #165975
doc

00:00:31

failed #165974
doc

00:00:37

failed #165973
doc

00:00:41

failed #165962
doc

00:00:20

failed #165892
doc

00:00:38

failed #165768
doc

00:00:37