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

utf8.h

@@ -1,6 +1,8 @@
 #ifndef GIT_UTF8_H
 #define GIT_UTF8_H
 
+struct strbuf;
+
 typedef unsigned int ucs_char_t;  /* assuming 32bit int */
 
 size_t display_mode_esc_sequence_len(const char *s);