replace: "libify" create_graft() and callees

replace: "libify" create_graft() and callees

File this away as yet another patch in the "libification" category.

As with all useful functions, in the next commit we want to use
create_graft() from a higher-level function where it would be
inconvenient if the called function simply die()s: if there is a
problem, we want to let the user know how to proceed, and the callee
simply has no way of knowing what to say.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>