git p4: Squash P4EDITOR in test harness

If P4EDITOR is set in the environment, test behavior could be
unpredictable. Set it explicitly.

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

t/lib-git-p4.sh

@@ -24,6 +24,7 @@ P4DPORT=$((10669 + ($testid - $git_p4_test_start)))
 
 export P4PORT=localhost:$P4DPORT
 export P4CLIENT=client
+export P4EDITOR=:
 
 db="$TRASH_DIRECTORY/db"
 cli="$TRASH_DIRECTORY/cli"