move load_ref_decorations() to log-tree.c and export it

log-tree.c is the ideal place for load_ref_decorations() and its
helper functions to live in, because the variable name_decoration
they're operating on is already located there, so move them thither.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.h

@@ -17,5 +17,6 @@ void log_write_email_headers(struct rev_info *opt, const char *name,
 			     const char **subject_p,
 			     const char **extra_headers_p,
 			     int *need_8bit_cte_p);
+void load_ref_decorations(void);
 
 #endif