git-p4: fix handling of multi-word P4EDITOR

git-p4: fix handling of multi-word P4EDITOR

This teaches git-p4 to pass the P4EDITOR variable to the
shell for expansion, so that any command-line arguments are
correctly handled. Without this, git-p4 can only launch the
editor if P4EDITOR is solely the path to the binary, without
any arguments.

This also adjusts t9805, which relied on the previous behaviour.

Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>