When testing on Travis, verify that tools/build was run

This is done by routing the generation of HTML into a temporary directory,
and checking its diff agains the existing public/ when running tests.

Fixes #237

tools/build

@@ -15,5 +15,27 @@ tools/format
 verbose && echo "Measuring line lengths..."
 tools/measure
 
-verbose && echo "Generating HTML..."
-tools/generate
+# SITE_DIR is the final location where we want generated content to be
+SITE_DIR="public"
+
+# GENERATE_DIR is where the content will be generated initially
+GENERATE_DIR="$(mktemp -d)"
+
+function cleanup() {
+  rm -rf "$GENERATE_DIR"
+}
+trap cleanup EXIT
+
+verbose && echo "Generating HTML to $GENERATE_DIR..."
+tools/generate $GENERATE_DIR
+
+# In TESTING mode, make sure that the generated content is identical to
+# what's already in SITE_DIR. If a difference is found, this script exits
+# with an error.
+if [[ ! -z "$TESTING" ]]; then
+	echo "Comparing $GENERATE_DIR with $SITE_DIR..."
+	diff -r "$GENERATE_DIR" "$SITE_DIR"
+fi
+
+verbose && echo "Copying $GENERATE_DIR to $SITE_DIR"
+cp -rf "${GENERATE_DIR}/." "$SITE_DIR"