Had to remove ${DIR} before renaming ${dir_new} to ${DIR}.
Also also add ${REV} to the info file, in case we need it in the future,
when extracting Zim's degit as a standalone tool for example.
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
