Several tools need to build a command that refers to a temporary

configuration file, to control tool behavior. To avoid a lot of repeated

work and the creation of many temporary files, some tools create part of

the command (and related files) up-front in handle_files() or

execute(). Others don't bother, and this can matter for larger

changes, slowing down execution or flooding the filesystem with

temporary files.
To standardize behavior, this change introduces a new

Tool.build_base_command() method. This will be called in execute(),

and the result will be passed on to handle_files() and

handle_file(), which can then build onto it to construct and execute

the command.
This is disabled for legacy tools.

Unit tests pass on Python 2.7 and 3.x.