git-update-dirs: Create and use get_config() to simplify things

b1d70ba4-ca5d-11e3-ae39-000df06acc56
git-update-dirs: Add three debug messages, available with -vv

8a4de246-ca5e-11e3-bb77-000df06acc56
git-update-dirs: Need to check for $Opt{'aggressive-compress'} too

Compression total stats weren't displayed if -C/--aggressive-compress
was used.

8678c3e2-ca73-11e3-ac36-000df06acc56
git-update-dirs: Make it possible to suppress commands with git config

If a git config variable on the format git-update-dirs.no-OPTION is set
to "true", the command will not be executed in this particular
repository.

4478dbb6-ca74-11e3-a2d9-000df06acc56
git-update-dirs: usage(): Move --lpar descpription to correct place

Or some (like git diff) would say that it's the --test description that
was misplaced. Potato, potato.

7a58f51c-ca75-11e3-b17e-000df06acc56