Fix "--pretty=format:" for parent related items.

There are two breakages in the %P/%p interpolation.  It appended
an excess SP at the end of the list, and it gave uninitialized
contents of a buffer on the stack for root commits.

This fixes it, while updating the t6006 test which expected the
wrong output.

Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t6006-rev-list-format.sh

@@ -39,11 +39,11 @@ EOF
 
 test_format parents %P%n%p <<'EOF'
 commit 131a310eb913d107dd3c09a65d1651175898735d
-86c75cfd708a0e5868dc876ed5b8bb66c80b4873 
-86c75cf 
+86c75cfd708a0e5868dc876ed5b8bb66c80b4873
+86c75cf
 commit 86c75cfd708a0e5868dc876ed5b8bb66c80b4873
-86c75cf 
-86c75cf 
+
+
 EOF
 
 # we don't test relative here