nfup_update: Add markdown formatting to the auto commit

After updating the lock file, reformat the commit message so that the
updates are wrapped in a code block, because that looks nicer.

Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>

bin/nix-flake-update-pr

@@ -13,6 +13,21 @@ nfup_setup_credentials() {
 
 nfup_update() {
     nix flake update --commit-lock-file
+
+    subject="$(git show -s --format=format:%s)"
+    message="$(git show -s --format=format:%b)"
+
+    preamble="$(echo "${message}" | head -n 1)"
+    update="$(echo "${message}" | tail -n +3)"
+
+    git commit --amend --message="${subject}
+
+${preamble}
+
+\`\`\`
+${update}
+\`\`\`
+"
 }
 
 nfup_push() {