"git diff <tree>{3,}": do not reverse order of arguments

According to the message of commit 0fe7c1de16,
"git diff" with three or more trees expects the merged tree first followed by
the parents, in order.  However, this command reversed the order of its
arguments, resulting in confusing diffs.  A comment /* Again, the revs are all
reverse */ suggested there was a reason for this, but I can't figure out the
reason, so I removed the reversal of the arguments.  Test case included.

Signed-off-by: Matt McCutchen <matt@mattmccutchen.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

builtin-diff.c

@@ -177,10 +177,8 @@ static int builtin_diff_combined(struct rev_info *revs,
 	if (!revs->dense_combined_merges && !revs->combine_merges)
 		revs->dense_combined_merges = revs->combine_merges = 1;
 	parent = xmalloc(ents * sizeof(*parent));
-	/* Again, the revs are all reverse */
 	for (i = 0; i < ents; i++)
-		hashcpy((unsigned char *)(parent + i),
-			ent[ents - 1 - i].item->sha1);
+		hashcpy((unsigned char *)(parent + i), ent[i].item->sha1);
 	diff_tree_combined(parent[0], parent + 1, ents - 1,
 			   revs->dense_combined_merges, revs);
 	return 0;

t/t4013-diff-various.sh

@@ -258,6 +258,7 @@ diff --patch-with-stat -r initial..side
 diff --patch-with-raw -r initial..side
 diff --name-status dir2 dir
 diff --no-index --name-status dir2 dir
+diff master master^ side
 EOF
 
 test_done

t/t4013/diff.diff_master_master^_side

@@ -0,0 +1,29 @@
+$ git diff master master^ side
+diff --cc dir/sub
+index cead32e,7289e35..992913c
+--- a/dir/sub
++++ b/dir/sub
+@@@ -1,6 -1,4 +1,8 @@@
+  A
+  B
+ +C
+ +D
+ +E
+ +F
++ 1
++ 2
+diff --cc file0
+index b414108,f4615da..10a8a9f
+--- a/file0
++++ b/file0
+@@@ -1,6 -1,6 +1,9 @@@
+  1
+  2
+  3
+ +4
+ +5
+ +6
++ A
++ B
++ C
+$