database: Store directory paths as relative, not absolute.

We were already storing relative mail filenames, so this is consistent
with that. Additionally, it means that directory documents remain
valid even if the database is relocated within its containing
filesystem.

lib/database.cc

@@ -86,7 +86,7 @@ typedef struct {
  *
  * The directory document is indexed with a single prefixed term:
  *
- *	directory:	The directory path (an absolute path)
+ *	directory:	The directory path (relative to the database path)
  *
  * and has a single value:
  *
@@ -639,6 +639,8 @@ notmuch_database_set_directory_mtime (notmuch_database_t *notmuch,
 	return NOTMUCH_STATUS_READONLY_DATABASE;
     }
 
+    path = _notmuch_database_relative_path (notmuch, path);
+
     db = static_cast <Xapian::WritableDatabase *> (notmuch->xapian_db);
     db_path = directory_db_path (path);
 

lib/notmuch.h

@@ -179,6 +179,11 @@ const char *
 notmuch_database_get_path (notmuch_database_t *database);
 
 /* Store an mtime within the database for 'path'.
+ *
+ * Here,'path' should be the path of a directory relative to the path
+ * of 'database' (see notmuch_database_get_path), or else should be an
+ * absolute path with initial components that match the path of
+ * 'database'.
  *
  * The intention is for the caller to use the mtime to allow efficient
  * identification of new messages to be added to the database. The