Use resolve_ref() to implement read_ref().

Symbolic refs are understood by resolve_ref(), so existing read_ref()
users will automatically understand them as well.

Signed-off-by: Junio C Hamano <junio@twinsun.com>
diff --git a/cache.h b/cache.h
index 958c96e..63823c3 100644
--- a/cache.h
+++ b/cache.h
@@ -230,6 +230,7 @@
 extern int get_sha1_hex(const char *hex, unsigned char *sha1);
 extern char *sha1_to_hex(const unsigned char *sha1);	/* static buffer result! */
 extern int read_ref(const char *filename, unsigned char *sha1);
+extern const char *resolve_ref(const char *path, unsigned char *sha1, int);
 
 /* General helper functions */
 extern void usage(const char *err) NORETURN;