Look for the last backup after removing old ones

With --remove-before-backup, the last backup was selected for
--link-dest before old backups were removed, so with keep=0 rsync
was given a directory that no longer existed.

simple_backup/simple_backup.py

@@ -515,7 +515,6 @@ class Backup:
         _notify('Starting backup...')
 
         self.define_backup_dir()
-        self.find_last_backup()
 
         fd, self._inputs_path = mkstemp(prefix='tmp_inputs', text=True)
         count = 0
@@ -549,6 +548,9 @@ class Backup:
         if self.keep != -1 and self._remove_before:
             self.remove_old_backups()
 
+        # Only look for the last backup now, as it may have been removed above
+        self.find_last_backup()
+
         logger.info('Copying files. This may take a long time...')
 
         if self._last_backup == '':