scripts/prepare_changelog: Update git log to only display PR merged commits

This change uses git flags to shorten the log messages to titles only, and uses the grep pattern on git to filter only commits that match merged commits, including squashed and merged commits.

scripts/prepare_changelog.sh

@@ -30,7 +30,7 @@ fi
 
 get_prs(){
     # git log v0.10.2...c3861d167533fb797b0fae0c380806625712e5f7 |
-    git log HEAD...${LAST_RELEASE} |
+    git log HEAD...${LAST_RELEASE} --first-parent --oneline --grep="Merge pull request #[0-9]\+" --grep="\(#[0-9]\+\)$" |
     grep -o "#\([0-9]\+\)" | awk -F\# '{print $2}' | while read line
     do
         grep -q "GH-${line}" CHANGELOG.md