Do not update commit time stamps while staging commits

Since change

 https://codereview.qt-project.org/gitweb?p=qtqa/gerrit-plugin-qt-workflow.git;a=commit;h=8da2a12a9cb4ebaf66607543e9a2379706e59d3d

has been merged to Gerrit, Coin is forced to unpack and repack source archives every time that they are created. The double-archiving is expensive operation that takes significant cpu time and exposes Coin to strange bugs like COIN-561.

I would suggest to amend the timestamp to the commit object (git hash-object) which will cause the commits to have different sha1s instead of tampering with the timestamps.