Escape arguments to filter commands

This will allow both the path to git-crypt and the path to the key file
to contain arbitrary characters, notably spaces.

util.hpp

@@ -38,6 +38,7 @@
 int		exec_command (const char* command, std::ostream& output);
 std::string	resolve_path (const char* path);
 void		open_tempfile (std::fstream&, std::ios_base::openmode);
+std::string	escape_shell_arg (const std::string&);
 
 #endif