Automatic bash completion

Automatic bash completion

Teach the build process to create a bash tab completion script. This
has three benefits:

  1. The tab completion gets faster, since it no longer has to run stg
     to figure out the set of available flags for each command.

  2. The tab completion script used to encode the list of stg
     subcommands, and the kind of arguments each subcommand expected.
     This information now lives in just one place: the subcommand's
     module.

  3. The tab completion script now knows what kind of argument each
     flag wants, and can tab complete those as well. So "stg refresh
     <tab>" will complete dirty files, but "stg refresh -p <tab>" will
     complete patch names.

Signed-off-by: Karl Hasselström <kha@treskal.com>