Only use core.askpass setting on git versions which support it

Git versions prior to 1.7.9 do not have the credentials support needed
for full git-client-plugin support. However, those older versions are
available in popular distributions like Red Hat Linux 5, Red Hat Linux 6,
and their derivatives like CentOS and Oracle Linux.

This change tries to avoid calling command line git with an argument
which those older versions do not support.