tests/test-serve.out

Infer a --repository argument from command arguments when reasonable.
In particular: if invoked without -R from a CWD not inside a repo, having been
passed one or more file paths as command arguments, where the nearest enclosing
repo of all of those paths is the same, quietly infer a -R option for that repo.
Otherwise abort with an error message as before.

% Without -v
access log created - .hg/hgrc respected
% With -v
listening at http://localhost/
% With --prefix foo
listening at http://localhost/foo/
% With --prefix /foo
listening at http://localhost/foo/
% With --prefix foo/
listening at http://localhost/foo/
% With --prefix /foo/
listening at http://localhost/foo/