Rename some test scripts and describe the naming convention
First digit: "family", e.g. the absolute basics and global stuff (0),
the basic db-side commands (read-tree, write-tree, commit-tree), the
basic working-tree-side commands (checkout-cache, update-cache), the
other basic commands (ls-files), the diff commands, the pull commands,
exporting commands, revision tree commands...
Second digit: the particular command we are testing
Third digit: (optionally) the particular switch or group of switches
we are testing