scm: mercurial: switch rev parameter of extension rhlog() if above Mercurial 1.6 or not
On Mercurial 1.5, following error raises.
<pre>
hg --config extensions.redminehelper=lib/redmine/scm/adapters/mercurial/redminehelper.py \
--rhbranch default --from default --to 0
abort: unknown revision '"default"'!
</pre>
