rust: add comment about lack of wdirrev handling
If hg is compiled with rust support, 'only(wdir())' crashed as
"rustext.GraphError: ('ParentOutOfRange',
2147483647)", which should instead
say "abort: working directory revision cannot be specified."
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.