Replace the +1 dep_index bias with ssize_t

This seems less likely to cause confusion.

I measured the run-time of initializing dep_index with both ordinary
stores and relaxed stores. On a 580MB deps log, the ordinary stores
(seq_cst) took about 4ms, while relaxed stores took 0.5ms.

Test: ninja_test
Change-Id: Id1a2cc026220e33b25c87dba4b5f612850e42bd0
1 file changed