index: correctly report which conflict stage has a wrong filemode
index: correctly report which conflict stage has a wrong filemode

When we're at offset 'i', we're dealing with the 'i+1' stage, since
conflicts start at 1.

leoyanggit/libgit2,joshtriplett/libgit2,stewid/libgit2,KTXSoftware/libgit2,KTXSoftware/libgit2,stewid/libgit2,yongthecoder/libgit2,saurabhsuniljain/libgit2,KTXSoftware/libgit2,saurabhsuniljain/libgit2,leoyanggit/libgit2,magnus98/TEST,stewid/libgit2,Tousiph/Demo1,Tousiph/Demo1,magnus98/TEST,Corillian/libgit2,joshtriplett/libgit2,stewid/libgit2,magnus98/TEST,stewid/libgit2,yongthecoder/libgit2,stewid/libgit2,magnus98/TEST,Tousiph/Demo1,KTXSoftware/libgit2,KTXSoftware/libgit2,jeffhostetler/public_libgit2,magnus98/TEST,joshtriplett/libgit2,yongthecoder/libgit2,leoyanggit/libgit2,magnus98/TEST,Corillian/libgit2,Corillian/libgit2,yongthecoder/libgit2,Corillian/libgit2,jeffhostetler/public_libgit2,joshtriplett/libgit2,joshtriplett/libgit2,Corillian/libgit2,jeffhostetler/public_libgit2,jeffhostetler/public_libgit2,yongthecoder/libgit2,jeffhostetler/public_libgit2,KTXSoftware/libgit2,jeffhostetler/public_libgit2,saurabhsuniljain/libgit2,Corillian/libgit2,Tousiph/Demo1,saurabhsuniljain/libgit2,yongthecoder/libgit2,leoyanggit/libgit2,leoyanggit/libgit2,Tousiph/Demo1,leoyanggit/libgit2,Tousiph/Demo1,saurabhsuniljain/libgit2,joshtriplett/libgit2,saurabhsuniljain/libgit2