reset_sdp.sh v2.6.8:
* Now clones SDP main branch from The Workshop by default, rather
than using a tar file.
* Added '-b' to override default branch (main), e.g. '-b dev'.
* Added '-t <tarfile>' (or '-t default') to use the old tarfile
install method of using a hard-coded tarball, good for when stability
is preferred over using the very latest.