{queue,song}_print: print relative paths if possible

If a song with an absolute path points inside the music directory,
print only the relative part.  This happens when partial songs from a
playlist file were loaded.

src/mapper.c

@@ -93,6 +93,16 @@ mapper_has_music_directory(void)
 	return music_dir != NULL;
 }
 
+const char *
+map_to_relative_path(const char *path_utf8)
+{
+	return music_dir != NULL &&
+		memcmp(path_utf8, music_dir, music_dir_length) == 0 &&
+		G_IS_DIR_SEPARATOR(path_utf8[music_dir_length])
+		? path_utf8 + music_dir_length + 1
+		: path_utf8;
+}
+
 char *
 map_uri_fs(const char *uri)
 {

src/mapper.h

@@ -41,6 +41,14 @@ void mapper_finish(void);
 bool
 mapper_has_music_directory(void);
 
+/**
+ * If the specified absolute path points inside the music directory,
+ * this function converts it to a relative path.  If not, it returns
+ * the unmodified string pointer.
+ */
+const char *
+map_to_relative_path(const char *path_utf8);
+
 /**
  * Determines the absolute file system path of a relative URI.  This
  * is basically done by converting the URI to the file system charset

src/queue_print.c

@@ -24,6 +24,7 @@
 #include "song_print.h"
 #include "locate.h"
 #include "client.h"
+#include "mapper.h"
 
 /**
  * Send detailed information about a range of songs in the queue to a

src/song_print.c

@@ -25,6 +25,7 @@
 #include "tag_print.h"
 #include "client.h"
 #include "uri.h"
+#include "mapper.h"
 
 void
 song_print_uri(struct client *client, struct song *song)
@@ -40,7 +41,8 @@ song_print_uri(struct client *client, struct song *song)
 		if (uri == NULL)
 			uri = song->uri;
 
-		client_printf(client, "%s%s\n", SONG_FILE, uri);
+		client_printf(client, "%s%s\n", SONG_FILE,
+			      map_to_relative_path(uri));
 
 		g_free(allocated);
 	}