commit.h: reduce unnecessary includes

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/merge.c b/merge.c
index da7fa65..10aaec3 100644
--- a/merge.c
+++ b/merge.c
@@ -7,6 +7,7 @@
 #include "commit.h"
 #include "run-command.h"
 #include "resolve-undo.h"
+#include "tree.h"
 #include "tree-walk.h"
 #include "unpack-trees.h"
 #include "dir.h"