Mark .gpg files in .git-crypt as binary

To remove any possibility of Git treating them as text by accident.

Closes AGWA#55.

commands.cpp

@@ -1202,6 +1202,7 @@ int add_gpg_user (int argc, const char** argv)
 		state_gitattributes_file << "# Do not edit this file.  To specify the files to encrypt, create your own\n";
 		state_gitattributes_file << "# .gitattributes file in the directory where your files are.\n";
 		state_gitattributes_file << "* !filter !diff\n";
+		state_gitattributes_file << "*.gpg binary\n";
 		state_gitattributes_file.close();
 		if (!state_gitattributes_file) {
 			std::clog << "Error: unable to write " << state_gitattributes_path << std::endl;