In VCSJobRunner::task() some execution paths left a signal/slot
connection, causing redundant output emission.
This would occur for example after a VCS job fails : if the next
job succeeds then its output is emitted twice, because the previous
connection for signal output() was not destroyed.
The bug is now fixed by ensuring disconnect is done for all execution
paths (thanks to helper DisconnectSignalHelper).

Change-Id: I67a7ba2829208f7e71158cb17a99575d79c9f9f7
Merge-request: 364
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-on: http://codereview.qt.nokia.com/2997