tools: unset git envvars in git-utils.sh
tools: unset git envvars in git-utils.sh

It would be nice to be able to jumpstart from a known point in the git
history by doing an interactive rebase and adding

  x tools/webpack-jumpstart

at the desired point.  Up until now that has been broken by git-rebase
setting a bunch of environment variables in the spawned command.  This
interferes with our use of git to work on the cache repository.

Unset those variables to fix the issue.

martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit