Rename some test scripts and describe the naming convention

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

Freeform part: commandname-details

Described in the README.

	mv t1000-checkout-cache.sh t2000-checkout-cache-clash.sh
	mv t1001-checkout-cache.sh t2001-checkout-cache-clash.sh
	mv t0200-update-cache.sh t2010-update-cache-badpath.sh
	mv t0400-ls-files.sh t3000-ls-files-others.sh
	mv t0500-ls-files.sh t3010-ls-files-killed.sh