commit: use timestamp_t for author_date_slab

The author_date_slab is used to store the author date of a commit when walking with the --author-date flag in rev-list or log. This was added as an 'unsigned long' in 81c6b38b "log: --author-date-order" Since 'unsigned long' is ambiguous in its bit-ness across platforms (64-bit in Linux, 32-bit in Windows, for example), most references to the author dates in commit.c were converted to timestamp_t in dddbad72 "timestamp_t: a new data type for timestamps" However, the slab definition was missed, leading to a mismatch in the data types in Windows. This would not reveal itself as a bug unless someone authors a commit after February 2106, but commits can store anything as their author date. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>