Create pack-write.c for common pack writing code

Create pack-write.c for common pack writing code

Include a generalized fixup_pack_header_footer() in this new file.
Needed by git-repack --max-pack-size feature in a later patchset.

[sp: Moved close(pack_fd) to callers, to support index-pack, and
     changed name to better indicate it is for packfiles.]

Signed-off-by: Dana L. How <redacted>
Signed-off-by: Shawn O. Pearce <redacted>