diff_tform: fix potential NULL pointer access
diff_tform: fix potential NULL pointer access

When the user passes in a diff which has no repository associated
we may call `git_config__get_int_force` with a NULL-pointer
configuration. Even though `git_config__get_int_force` is
designed to swallow errors, it is not intended to be called with
a NULL pointer configuration.

Fix the issue by only calling `git_config__get_int_force` only
when configuration could be retrieved from the repository.

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