VCS[hg]: Mercurial forgets working directory from commit #2 on.

Do not clear variable containing submit repository. Fix potential
crash (closing diff editor before command termination),
use proper temp file name. Pass --non-interactive to "commit"
to suppress editors being launched (despite -l).

Reviewed-by: Marco Bubke <marco.bubke@nokia.com>
Task-number: QTCREATORBUG-1503