object-name.h: move declarations for object-name.c functions from cache.h

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/combine-diff.c b/combine-diff.c
index 6586e45..cd5f39f 100644
--- a/combine-diff.c
+++ b/combine-diff.c
@@ -7,6 +7,7 @@
 #include "diffcore.h"
 #include "environment.h"
 #include "hex.h"
+#include "object-name.h"
 #include "quote.h"
 #include "xdiff-interface.h"
 #include "xdiff/xmacros.h"