diff: ignore nsecs when diffing
diff: ignore nsecs when diffing

Although our index contains the literal time present in the index,
we do not read nanoseconds from disk, and thus we should not use
them in any comparisons, lest we always think our working directory
is dirty.

Guard this behind a `GIT_USE_NSECS` for future improvement.

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