testing: Exclude files that are ignored in Git from the distribution

Since the complete hosts and tests directories are part of the tarball
this would include generated certificates and keys.

testing/Makefile.am

@@ -2,3 +2,11 @@ EXTRA_DIST = do-tests make-testing ssh start-testing stop-testing \
              testing.conf ssh_config config hosts images scripts tests \
 			 README
 
+# exclude all files ignored by Git from the tarball
+dist-hook:
+	if git -C $(srcdir) ls-files --error-unmatch hosts > /dev/null 2>&1; then \
+		find $(abs_srcdir)/hosts $(abs_srcdir)/tests -print0 | \
+			sed -z -e "s|^$(abs_srcdir)/||" | \
+			git -C $(srcdir) check-ignore -z --stdin | \
+			xargs -0 -I{} rm -rf $(distdir)/{}; \
+	fi