repo: look up nullrev context by revnum, not symbolic name
I think lookup of the 'null' symbol should be done via
scmutil.revsymbol() and repo['null'] is only supported for historical
reasons. However, repo[nullrev] is fine, so we can switch to that
instead of switching to scmutil.revsymbol('null').
Differential Revision: https://phab.mercurial-scm.org/D4780
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.