commit.h: reduce unnecessary includes

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge-tree.c

@@ -16,6 +16,7 @@
 #include "exec-cmd.h"
 #include "merge-blobs.h"
 #include "quote.h"
+#include "tree.h"
 
 static int line_termination = '\n';