git-p4 will just halt if there is not enough disk space while
streaming content from P4 to Git. Add a check to ensure at least
4 times (arbitrarily chosen) the size of a streamed file is available.
Signed-off-by: Lars Schneider <larsxschneider@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>