localrepo: use revsymbol() in lookup()
lookup() seems to be about looking up a revision based on a symbol
that may come from the user (via the wire protocol), so revsymbol() is
appropriate here.
Differential Revision: https://phab.mercurial-scm.org/D3055
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.