Add script to verify all merge commits are signed

contrib/verify-commits/pre-push-hook.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+if ! [[ "$2" =~ [git@]?[www.]?github.com[:|/]bitcoin/bitcoin[.git]? ]]; then
+    exit 0
+fi
+
+while read LINE; do
+    set -- A $LINE
+    if [ "$4" != "refs/heads/master" ]; then
+        continue
+    fi
+    if ! ./contrib/verify-commits/verify-commits.sh $3 > /dev/null 2>&1; then
+        echo "ERROR: A commit is not signed, can't push"
+        ./contrib/verify-commits/verify-commits.sh
+        exit 1
+    fi
+done < /dev/stdin