ref-filter: add function to print single ref_array_item

ref-filter functions are useful for printing git object information
using a format specifier. However, some other modules may not want to use
this functionality on a ref-array but only print a single item.

Expose a pretty_print_ref function to create, pretty print and free
individual ref-items.

Signed-off-by: Lukas Puehringer <luk.puehringer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.h

@@ -107,4 +107,11 @@ struct ref_sorting *ref_default_sorting(void);
 /*  Function to parse --merged and --no-merged options */
 int parse_opt_merge_filter(const struct option *opt, const char *arg, int unset);
 
+/*
+ * Print a single ref, outside of any ref-filter. Note that the
+ * name must be a fully qualified refname.
+ */
+void pretty_print_ref(const char *name, const unsigned char *sha1,
+		const char *format);
+
 #endif /*  REF_FILTER_H  */