entry.c: convert strcpy to xsnprintf

entry.c: convert strcpy to xsnprintf

This particular conversion is non-obvious, because nobody
has passed our function the length of the destination
buffer. However, the interface to checkout_entry specifies
that the buffer must be at least TEMPORARY_FILENAME_LENGTH
bytes long, so we can check that (meaning the existing code
was not buggy, but merely worrisome to somebody reading it).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>