fix thumbnail generation on every push

post-receive git hook is also tweaked. since thumbnails aren't needed on
site build time, they can be created after by copying `thumbs.sh` script
into site art directory, running it and deleting afterwards.

`hooks/post-receive`
```

{
    cp "$TMP_GIT_CLONE/art/thumbs.sh" "$PUBLIC_WWW/art/thumbs.sh"
    cd "$PUBLIC_WWW/art"
    sh thumbs.sh
    rm thumbs.sh
}

```

1 files changed, 5 insertions, 1 deletions

diff --git a/art/thumbs.sh b/art/thumbs.sh
index 758b0a9..71be5b4 100755
--- a/art/thumbs.sh
+++ b/art/thumbs.sh
@@ -30,7 +30,11 @@ do
 
     [ -d "$_thumbs_dir" ] || mkdir "$_thumbs_dir"
 
-    _imgs="$(find . -maxdepth 1 -type f | cut -b 3-)"
+    _imgs="$(find . -maxdepth 1 \
+        -iname "*.jpg" -or \
+        -iname "*.jpeg" -or \
+        -iname "*.gif" -or \
+        -iname "*.png" -type f | cut -b 3-)"
     for _img in $_imgs
     do
         _ext="${_img##*.}"