improve git ignoring of build paths
improve git ignoring of build paths

halfflat/arbor,halfflat/arbor,halfflat/nestmc-proto,halfflat/arbor,halfflat/arbor,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/nestmc-proto,eth-cscs/nestmc-proto,halfflat/nestmc-proto,eth-cscs/nestmc-proto