LU-7968 lbuild: Fix reuse signature calculation

LU-7968 lbuild: Fix reuse signature calculation

Don't use an information about Lustre commits from target file content
that is not directly related to kernel build which should be cached.
In other words make signature independent from git commits hash.

Change-Id: I3dbeb5f3a94cb947221aaa92659f14d4ea65e30a
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: http://review.whamcloud.com/19262
Reviewed-by: Doug Oucharek <doug.s.oucharek@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Tested-by: Jenkins
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>