merge: allow a custom working context to be passed to update
This will allow anyone to enable the first in-menmory merge milestone
by wrapping merge.update in an extension and creating an overlayworkingctx.
Differential Revision: https://phab.mercurial-scm.org/D682
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.