plumbing/format: idxfile, unsigned values are never < 0
plumbing/format: idxfile, unsigned values are never < 0

Signed-off-by: Christian Muehlhaeuser <0a362b7528cb3a818f8ef2ffbc5db2ca2860109d@gmail.com>
(cherry picked from commit 047bb4f6d0657389ddd4ca3230ff3bee08d66a6b)

src-d/go-git,mcuadros/go-git,go-git/go-git,src-d/go-git,mcuadros/go-git,go-git/go-git