fix missing error in bfsCommitIterator
fix missing error in bfsCommitIterator

Signed-off-by: niukuo <13aa5ecf20b1c0cd0732191aab21a06846ae527a@gmail.com>

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