Merge branch 'en/incl-forward-decl'

Code hygiene improvement for the header files.

* en/incl-forward-decl:
  Remove forward declaration of an enum
  compat/precompose_utf8.h: use more common include guard style
  urlmatch.h: fix include guard
  Move definition of enum branch_track from cache.h to branch.h
  alloc: make allocate_alloc_state and clear_alloc_state more consistent
  Add missing includes and forward declarations

diffcore.h

@@ -4,6 +4,10 @@
 #ifndef DIFFCORE_H
 #define DIFFCORE_H
 
+#include "cache.h"
+
+struct diff_options;
+
 /* This header file is internal between diff.c and its diff transformers
  * (e.g. diffcore-rename, diffcore-pickaxe).  Never include this header
  * in anything else.