Create symlinks in t/configure-sampledata; remove them from git.

Since git doesn't always handle symlinks the way bup expects (Cygwin),
don't keep them in git, create them via "t/configure-sampledata
--setup".

This also provides a place to handle anything else that falls outside
git's purview that we might want to add.

Integrate configure-sampledata into the makefile clean and all
targets.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>