fix(contrib): Fix markdown output of changelog script

Now the script will generate markdown lists just as the current
changelog in deis/deis:CHANGELOG.md is in.
This patch seems to fix that erratic behaviour with `--pretty`:

```
diff --git a/contrib/util/generate-changelog.sh b/contrib/util/generate-changelog.sh
index 3f1ace4..575b7ea 100755
--- a/contrib/util/generate-changelog.sh
+++ b/contrib/util/generate-changelog.sh
@@ -5,13 +5,13 @@ usage() {
 }

 retrieve() {
-    git --no-pager log --oneline --no-merges --oneline --pretty=format:" - %h %s" --grep="$1" $FROM
+    git --no-pager log --oneline --no-merges --oneline --format=" - %h %s" --grep="$1" $FROM..$TO
 }

 subheading() {
     echo "#### $1\n"
     retrieve "$2"
-    echo "\n" # For some reason with the pretty format you need an extra line here
+    echo
 }

 FROM=$1
```