text_file: allocate line buffers dynamically

Use a single GString buffer object in all functions loading the
database.  Enlarge it automatically for long lines.  This eliminates
the maximum line length for tag values.  There is still an upper limit
of 512 kB to prevent denial of service, but that's reasonable I guess.

src/directory_save.h

@@ -31,6 +31,7 @@ int
 directory_save(FILE *fp, struct directory *directory);
 
 bool
-directory_load(FILE *fp, struct directory *directory, GError **error);
+directory_load(FILE *fp, struct directory *directory,
+	       GString *buffer, GError **error);
 
 #endif