vc-list-files: avoid failure when /bin/sh is dash

vc-list-files: avoid failure when /bin/sh is dash

* build-aux/vc-list-files: Avoid a shell portability problem with dash.
On some Debian based systems, /bin/sh is a symlink to dash, and running
this command would omit the "/" following each 'tests' prefix:
  dash -x build-aux/vc-list-files -C . tests
That is because bash and dash work differently:
  $ for i in bash dash; do $i -c 'a=odd; a=ok b=$a; echo '$i' $b'; done
  bash ok
  dash odd