Do not generate ChangeLog when outside a Git repo

The script "build-aux/genChangelog" needs to be executed from within a
Git repository; if it's not, it should not be executed.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>