don't rebuild tags if they were not involved at all

(otherwise posts_with_tags would endlessly freeze)

bb.sh

@@ -408,8 +408,10 @@ edit() {
     echo "Posted $filename"
     tags_after="$(tags_in_post $filename)"
     relevant_tags="$(echo "$tags_before $tags_after" | tr ' ' '\n' | sort -u | tr '\n' ' ')"
-    relevant_posts="$(posts_with_tags $relevant_tags) $filename"
-    rebuild_tags "$relevant_posts" "$relevant_tags"
+    if [ "$relevant_tags" ]; then
+        relevant_posts="$(posts_with_tags $relevant_tags) $filename"
+        rebuild_tags "$relevant_posts" "$relevant_tags"
+    fi
 }
 
 # Adds the code needed by the twitter button
@@ -680,8 +682,10 @@ EOF
     chmod 644 "$filename"
     echo "Posted $filename"
     relevant_tags="$(tags_in_post $filename)"
-    relevant_posts="$(posts_with_tags $relevant_tags) $filename"
-    rebuild_tags "$relevant_posts" "$relevant_tags"
+    if [ "$relevant_tags" ]; then
+        relevant_posts="$(posts_with_tags $relevant_tags) $filename"
+        rebuild_tags "$relevant_posts" "$relevant_tags"
+    fi
 }
 
 # Create an index page with all the posts