We use `git describe --tags` to get information about the number of commit since
the last major version, and the sha of the current HEAD. This is added to the
ZULIP_VERSION when a deploy is done from `git`.
Modified heavily by punchagan to:
* to use git describe instead of `git log` and `wc`
* use a separate script to run the git describe command
* write the file with version info to var/ and remove it from the repo
Fixes#4685.
