autogen.sh: Change CVS to Git in error message
autogen.sh: Change CVS to Git in error message

GNOME has migrated to Git, so the error message should no longer refer
to the GNOME CVS.

https://bugzilla.gnome.org/show_bug.cgi?id=730340

Distrotech/gcr,Distrotech/gcr,Distrotech/gcr,Distrotech/gcr