Organize repo into per-command subdirectories

* put each helper in a directory (usually) named after the `git` command that it is most directly augmenting
* split various `alias` definitions into per-command/subdir `.git-rc` files that are `source`d from the main `.git-rc`
* various scripts for manipulating this repo were placed in `admin`
* added some scaffolding for writing shell unit tests of various scripts with the addition of `nav/test`
do you want to run admin/find-broken-links here, too?