and also simplify code by setting multiple variables at once. This
should make the resulting script a tiny bit smaller, while not
compromising on legibility.
and rename previous to git.
Tools allow for different install and update scripts. The git tool does
what the previous implementation did (installs and updates using the git
client). The new tool installs and updates with the GitHub API, not
using the git client, but using curl/wget and tar instead.
Closes#373