blame_git: handle error returned by `git_commit_parent`
blame_git: handle error returned by `git_commit_parent`

joshtriplett/libgit2,stewid/libgit2,saurabhsuniljain/libgit2,Corillian/libgit2,saurabhsuniljain/libgit2,stewid/libgit2,leoyanggit/libgit2,leoyanggit/libgit2,stewid/libgit2,saurabhsuniljain/libgit2,Corillian/libgit2,joshtriplett/libgit2,leoyanggit/libgit2,jeffhostetler/public_libgit2,jeffhostetler/public_libgit2,joshtriplett/libgit2,leoyanggit/libgit2,jeffhostetler/public_libgit2,stewid/libgit2,jeffhostetler/public_libgit2,joshtriplett/libgit2,leoyanggit/libgit2,Corillian/libgit2,leoyanggit/libgit2,stewid/libgit2,Corillian/libgit2,jeffhostetler/public_libgit2,joshtriplett/libgit2,stewid/libgit2,joshtriplett/libgit2,saurabhsuniljain/libgit2,jeffhostetler/public_libgit2,Corillian/libgit2,Corillian/libgit2,saurabhsuniljain/libgit2,saurabhsuniljain/libgit2