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.

Thanks to @bacongobbler for the insight about `--format` over
`--pretty=format` and unneeded newlines not being echoed.

Author: Xe

contrib/util/generate-changelog.sh

@@ -5,7 +5,7 @@ usage() {
 }
 
 retrieve() {
-    git --no-pager log --oneline --no-merges --grep="$1" $FROM..$TO
+    git --no-pager log --oneline --no-merges --oneline --format=" - %h %s" --grep="$1" $FROM..$TO
 }
 
 subheading() {