Improve gitattributes (#114)

Author: timacdonald

.gitattributes

@@ -11,11 +11,11 @@
 /.gitattributes export-ignore
 /.github export-ignore
 /.gitignore export-ignore
-/agent/composer.json
-/agent/composer.lock
+/agent/composer.json export-ignore
+/agent/composer.lock export-ignore
 /art export-ignore
-/client/composer.json
-/client/composer.lock
+/client/composer.json export-ignore
+/client/composer.lock export-ignore
 /composer-dependency-analyser.php export-ignore
 /phpstan.neon.dist export-ignore
 /phpunit.xml.dist export-ignore