Fix dirty snap release.

https://stackoverflow.com/questions/16035240/why-is-git-describe-dirty-adding-a-dirty-suffix-when-describing-a-clean-ch
https://forum.snapcraft.io/t/dirty-snap-release/12975

Thanks to xet7.

snapcraft.yaml

@@ -1,6 +1,6 @@
 name: wekan
 version: 0
-version-script: git describe --dirty --tags |  cut -c 2-
+version-script: git update-index --refresh && git describe --dirty --tags |  cut -c 2-
 summary: The open-source kanban
 description: |
    Wekan is an open-source and collaborative kanban board application.