git-p4: Support purged files and optimize memory usage

git-p4: Support purged files and optimize memory usage

Purged files are handled as if they are merely deleted, which is not
entirely optimal, but I don't know of any other way to handle them.
File data is deleted from memory as early as they can, and they are more
efficiently handled, at (significant) cost to CPU usage.

Still need to handle p4 branches with spaces in their names.
Still need to make git-p4 clone more reliable.
 - Perhaps with a --continue option. (Sometimes the p4 server kills
 the connection)

Signed-off-by: John Chapman <thestar@fussycoder.id.au>
Acked-by: Simon Hausmann <simon@lst.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>