Installation script does not fetch git tags. Documentation does not reflect this.
### Expected behavior.

Documentation says:
"By default the latest version of SpaceVim will be installed. If you want to switch to specific version, for example v1.8.0, run following command in your terminal.
cd ~/.SpaceVim
git checkout v1.8.0"

However the installation script does not fetch tags from git repository, so git cannot checkout unknown tag. That is easy to fix if user knows git, but hard for other.
Fix:
cd ~/.SpaceVim
git fetch --tags
git checkout v1.8.0

### The reproduce ways from Vim starting.

curl -sLf https://spacevim.org/install.sh | bash
cd ~/.SpaceVim
git checkout v1.8.0

### Output of the `:SPDebugInfo!`

not relevant