Introduce mutation test script for changed code (#757)

While there, optimize the existing `run-mutation-tests.sh` script.

run-branch-mutation-tests.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+# Executes Pitest to determine the mutation test coverage of changed code. By
+# default code that is changed relative to the default branch is exercised,
+# though another change set may be specified; see the Arcmutate Git integration
+# documentation for details [1]. The results are found in each Maven module's
+# `target/pit-reports` directory.
+#
+# [1] https://docs.arcmutate.com/docs/git-integration.html
+
+set -e -u -o pipefail
+
+if [ "${#}" -gt 1 ]; then
+  echo "Usage: ${0} [DiffSpec]"
+  exit 1
+fi
+
+diffSpec="${1:-+GIT(from[refs/remotes/origin/HEAD])}"
+
+mvn clean test-compile pitest:mutationCoverage \
+  -DargLine.xmx=2048m \
+  -Dverification.skip \
+  -Dfeatures="${diffSpec}"