commit.h: rearrange 'index' to shrink struct commit

commit.h: rearrange 'index' to shrink struct commit

On linux 64-bit architecture, pahole finds that there's a 4 bytes
padding after 'index'. Moving it to the end reduces this struct's size
from 72 to 64 bytes (because of another 4 bytes padding after
graph_pos). On linux 32-bit, the struct size remains 52 bytes like
before.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Junio C Hamano <redacted>