cmdutil: reimplement finddate() without using walkchangerevs()
It's simpler and slightly faster maybe because a fewer Python ops would
run.
Unscientific benchmark:
$ python -m timeit \
-s 'from mercurial import hg, ui, cmdutil; repo = hg.repository(ui.ui())' \
'cmdutil.finddate(repo.ui, repo, "<2008-01-01")'
(orig) 10 loops, best of 3: 1.45 sec per loop
(new) 10 loops, best of 3: 1.25 sec per loop
Now "hg churn" and "hg grep" are the only users of walkchangerevs(), which
I want to refactor and fix bugs.
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