Add mapmerge 2 (#4645)

Adds TG's mapmerge2, a new TGM-compatible merger that handles things like dictionary expansion much better than the old tool, as well as supporting a git merge hook for map files.

This PR does not remove the old merger as this one is more annoying to setup.

tools/hooks/python.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+set -e
+if command -v python3 >/dev/null 2>&1; then
+	PY=python3
+else
+	PY=python
+fi
+PATHSEP=$($PY - <<'EOF'
+import sys, os
+if sys.version_info.major != 3 or sys.version_info.minor < 6:
+	sys.stderr.write("Python 3.6+ is required: " + sys.version + "\n")
+	exit(1)
+print(os.pathsep)
+EOF
+)
+export PYTHONPATH=tools/mapmerge2/${PATHSEP}${PYTHONPATH}
+$PY "$@"