merge: don't grab wlock when merging in memory
I noticed this because we have an internal extension that does an
in-memory rebase while holding only a repo lock, which resulted in a
developer warning about the working copy lock being taken after the
repo lock.
Differential Revision: https://phab.mercurial-scm.org/D8665
lib - sys sys
mercurial - sys sys
hgrc.d - sys sys
9diff.rc - sys sys
factotum.rc - sys sys
rc - sys sys
bin - sys sys
9diff - sys sys
hg - sys sys
sys - sys sys
lib - sys sys
python - sys sys
lib - sys sys
python2.5 - sys sys
site-packages - sys sys
hgext - sys sys
+ - sys sys
mercurial - sys sys
+ - sys sys
mercurial-VERSION-py2.5.egg-info - sys sys
src - sys sys
cmd - sys sys
hg - sys sys
+ - sys sys