Don't run "git describe" in get_version.sh.

Instead of leaving snapshot_version empty in VERSION, set it to
"gitclone" so that a) we don't have to have replicated "git describe"
logic in get_version.sh, and b) ./configure on a developer's git
checkout will get a non-empty value.

This slightly simplifies the logic in get_version.sh, and, more
importantly, makes it easier for us to maintain because "git describe"
is now only invoked from a single place: distscript.csh (i.e., during
"make dist").