fast-import: use sha1write() for pack data
fast-import: use sha1write() for pack data

This is in preparation for using write_idx_file().  Also, by using
sha1write() we get some buffering to reduces the number of write
syscalls, and the written data is SHA1 summed which allows for the extra
data integrity validation check performed in fixup_pack_header_footer()
(details on this in commit abeb40e5aa).

Signed-off-by: Nicolas Pitre <d659c10e27d52b00987b65e85d99bce5480adcae@fluxnic.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git