sha1dc_git.h: re-arrange an ifdef chain for a subsequent change

A subsequent change will change the semantics of DC_SHA1_SUBMODULE in
a way that would require moving these checks around, so start by
moving them around without any functional changes to reduce the size
of the subsequent patch.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1dc_git.h

@@ -1,9 +1,9 @@
 /* Plumbing with collition-detecting SHA1 code */
 
-#ifdef DC_SHA1_SUBMODULE
-#include "sha1collisiondetection/lib/sha1.h"
-#elif defined(DC_SHA1_EXTERNAL)
+#ifdef DC_SHA1_EXTERNAL
 #include <sha1dc/sha1.h>
+#elif defined(DC_SHA1_SUBMODULE)
+#include "sha1collisiondetection/lib/sha1.h"
 #else
 #include "sha1dc/sha1.h"
 #endif