Reuse sha1 in packed_git in fast-import.

Reuse sha1 in packed_git in fast-import.

Rather than maintaing our own packfile level sha1 variable we
can make use of the one already available in struct packed_git.
Its meant for the SHA1 of the index but it can also hold the
SHA1 of the packfile itself between final checksumming of the
packfile and creation of the index.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>