rewrite vmod_abi.h / vcs_version.h only if git hash changed
rewrite vmod_abi.h / vcs_version.h only if git hash changed

This got lost with a29fca70f7ccc75964bcfffb8c8ab1617fcf2bba

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache