directory: added "mtime" property

Remember the modification time of each directory.  This is important
for archives (which are virtual directories right now), but may also
be useful for an automatic update mechanism.

src/directory_save.c

@@ -28,7 +28,7 @@
 #include <assert.h>
 #include <string.h>
 
-#define DIRECTORY_MTIME "mtime: " /* DEPRECATED, noop-read-only */
+#define DIRECTORY_MTIME "mtime: "
 #define DIRECTORY_BEGIN "begin: "
 #define DIRECTORY_END "end: "
 
@@ -41,6 +41,9 @@ directory_save(FILE *fp, struct directory *directory)
 	int retv;
 
 	if (!directory_is_root(directory)) {
+		fprintf(fp, DIRECTORY_MTIME "%lu\n",
+			(unsigned long)directory->mtime);
+
 		retv = fprintf(fp, "%s%s\n", DIRECTORY_BEGIN,
 			       directory_get_path(directory));
 		if (retv < 0)
@@ -84,8 +87,12 @@ directory_load(FILE *fp, struct directory *directory)
 			strcpy(key, &(buffer[strlen(DIRECTORY_DIR)]));
 			if (!fgets(buffer, sizeof(buffer), fp))
 				FATAL("Error reading db, fgets\n");
-			/* for compatibility with db's prior to 0.11 */
+
 			if (g_str_has_prefix(buffer, DIRECTORY_MTIME)) {
+				directory->mtime =
+					g_ascii_strtoull(buffer + sizeof(DIRECTORY_MTIME) - 1,
+							 NULL, 10);
+
 				if (!fgets(buffer, sizeof(buffer), fp))
 					FATAL("Error reading db, fgets\n");
 			}