version: Show number of commits and commit sha in ZULIP_VERSION.

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.

scripts/lib/upgrade-zulip-stage-2

@@ -120,6 +120,8 @@ elif args.from_git:
     subprocess.check_call(["./tools/update-prod-static", "--authors-not-required", "--prev-deploy",
                            os.path.join(DEPLOYMENTS_DIR, 'current')],
                           preexec_fn=su_to_zulip)
+    logging.info("Caching zulip git version...")
+    subprocess.check_call(["./tools/cache-zulip-git-version"], preexec_fn=su_to_zulip)
 else:
     # Since this doesn't do any actual work, it's likely safe to have
     # this run before we apply puppet changes (saving a bit of downtime).