only re-generate the tags that have changed

When I wrote a new blog post, it should have a tag (aka. category). My tags are managed using file system as the database: every tag is a filesystem directory, and every post of this tag is symlinked into this directory.

When I publish a post, I should only re-generate the tags that are affected, instead of all tags of my blog system: 

diff --git a/generate-tags.sh b/generate-tags.sh
index bb579c3..ef42695 100755
--- a/generate-tags.sh
+++ b/generate-tags.sh
@@ -66,4 +66,5 @@ for tagd in meta/tags/*/; do
     ) > $(dirname $tagd)/$tag.org
 done
 
-org-export -L meta/tags/*.org
+
+org-export -L $(git st -s meta/tags/*.org|pn 2)