object-store: move lookup_replace_object to replace-object.h

lookup_replace_object is a low-level function that most users of the
object store do not need to use directly.

Move it to replace-object.h to avoid a dependency loop in an upcoming
change to its inline definition that will make use of repository.h.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/mktag.c b/builtin/mktag.c
index 9f5a50a..e3d20a7 100644
--- a/builtin/mktag.c
+++ b/builtin/mktag.c
@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "tag.h"
+#include "replace-object.h"
 
 /*
  * A signature file has a very simple fixed format: four lines