Add wrapper script for selecting the right deployment key

See https://www.webfactory.de/blog/using-multiple-ssh-deploy-keys-with-github.

wrapper/ssh-deploy-key-wrapper.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+# The last argument is the command to be executed on the remote end, which is something
+# like "git-upload-pack 'webfactory/ssh-agent.git'". We need the repo path only, so we
+# loop over this last argument to get the last part of if.
+for last in ${!#}; do :; done
+
+# Don't use "exec" to run "ssh" below; then the trap won't work.
+key_file=$(mktemp -u)
+trap "rm -f $key_file" EXIT
+
+eval last=$last
+
+# Try to pick the right key
+ssh-add -L | grep --word-regexp --max-count=1 $last > $key_file
+
+ssh -i $key_file "$@"