Coverity currently lists a lot of errors with regard to
GITERR_CHECK_ALLOC causing resource leaks. We know this macro is
only invoked when we want to abort because we are out of memory.

Coverity allows for overriding the default model where we know
that certain functions guarantee a desired behavior. The
user_nodefs.h is used to override the behavior of macros.
Re-define GITERR_CHECK_ALLOC inside of it to specify its abort
nature.