Generalize the "show each ref" code in receice-pack

This turns it into a generic "do xyz for each ref" library function.

refs.h

@@ -1,6 +1,12 @@
 #ifndef REFS_H
 #define REFS_H
 
+/*
+ * Calls the specified function for each ref file until it returns nonzero,
+ * and returns the value
+ */
+extern int for_each_ref(int (*fn)(const char *path, unsigned char *sha1));
+
 /** Reads the refs file specified into sha1 **/
 extern int get_ref_sha1(const char *ref, unsigned char *sha1);