fast-import: simplify allocation in start_packfile

fast-import: simplify allocation in start_packfile

This function allocate a packed_git flex-array, and adds a
mysterious 2 bytes to the length of the pack_name field. One
is for the trailing NUL, but the other has no purpose. This
is probably cargo-culted from add_packed_git, which gets the
".idx" path and needed to allocate enough space to hold the
matching ".pack" (though since 48bcc1c, we calculate the
size there differently).

This site, however, is using the raw path of a tempfile, and
does not need the extra byte. We can just replace the
allocation with FLEX_ALLOC_STR, which handles the allocation
and the NUL for us.

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