build: add excludes in gitattributes

Signed-off-by: Bart Jaskulski <bjaskulski@protonmail.com>

.gitattributes

+# Path-based git attributes
+# https://www.kernel.org/pub/software/scm/git/docs/gitattributes.html
+
+# Ignore all test and documentation with "export-ignore".
+/.editorconfig               export-ignore
+/.gitattributes              export-ignore
+/.gitignore                  export-ignore
+/.gitlab-ci.yml              export-ignore
+/phpcs.xml.dist              export-ignore
+/phpunit-unit.xml            export-ignore
+/phpunit-integration.xml     export-ignore
+/tests                       export-ignore