Avoid updating zfs_gitrev.h when rev is unchanged

Build process would always re-compile spa_history.c due to touching
zfs_gitrev.h - avoid if no change in gitrev.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Allan Jude <allanjude@freebsd.org>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #8860