ls: reduce memory consumption and speed up (don't do redundant [l]stat)

ls: reduce memory consumption and speed up (don't do redundant [l]stat)

function                                             old     new   delta
my_stat                                              110     278    +168
splitdnarray                                         120     118      -2
ls_main                                              769     765      -4
list_single                                          961     931     -30
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(add/remove: 0/0 grow/shrink: 1/3 up/down: 168/-36)           Total: 132 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>