diff: use `context.diff` to produce diff
We want to make sure `context.diff` if full featured. Using it in the main
diff code path is a good way to do so.
The end goal is to be able to easily compute diff between in-memory context.
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.