config: copy the return value of prefix_filename

config: copy the return value of prefix_filename

The prefix_filename function returns a pointer to a static
buffer which may be overwritten by subsequent calls. Since
we are going to keep the result around for a while, let's be
sure to duplicate it for safety.

I don't think this can be triggered as a bug in the current
code, but it's a good idea to be defensive, as any resulting
bug would be quite subtle.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>