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: NJunio C Hamano <junkio@cox.net>

commit.c

@@ -849,19 +849,23 @@ static long format_commit_message(const struct commit *commit,
 	interp_set_entry(table, ITREE_ABBREV,
 			find_unique_abbrev(commit->tree->object.sha1,
 				DEFAULT_ABBREV));
+
+	parents[1] = 0;
 	for (i = 0, p = commit->parents;
 			p && i < sizeof(parents) - 1;
 			p = p->next)
-		i += snprintf(parents + i, sizeof(parents) - i - 1, "%s ",
+		i += snprintf(parents + i, sizeof(parents) - i - 1, " %s",
 			sha1_to_hex(p->item->object.sha1));
-	interp_set_entry(table, IPARENTS, parents);
+	interp_set_entry(table, IPARENTS, parents + 1);
+
+	parents[1] = 0;
 	for (i = 0, p = commit->parents;
 			p && i < sizeof(parents) - 1;
 			p = p->next)
-		i += snprintf(parents + i, sizeof(parents) - i - 1, "%s ",
+		i += snprintf(parents + i, sizeof(parents) - i - 1, " %s",
 			find_unique_abbrev(p->item->object.sha1,
 				DEFAULT_ABBREV));
-	interp_set_entry(table, IPARENTS_ABBREV, parents);
+	interp_set_entry(table, IPARENTS_ABBREV, parents + 1);
 
 	for (i = 0, state = HEADER; msg[i] && state < BODY; i++) {
 		int eol;

t/t6006-rev-list-format.sh

@@ -42,8 +42,8 @@ commit 131a310eb913d107dd3c09a65d1651175898735d
 86c75cfd708a0e5868dc876ed5b8bb66c80b4873
 86c75cf
 commit 86c75cfd708a0e5868dc876ed5b8bb66c80b4873
-86c75cf 
-86c75cf 
+
+
 EOF
 
 # we don't test relative here