release_tool: Use temp directory for modifications, not user's worktree.
release_tool: Use temp directory for modifications, not user's worktree.

This way we can follow remote branches by default for all
repositories, instead of having integration be an exception because we
need to modify its docker-compose files. Now we check out into a temp
directory instead, make the modifications there, and then pull back
the changes into the original repository.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@mender.io>
(cherry picked from commit 20a64a8bd9e654a3a9d299814db8bade140acd95)

pasinskim/integration,GregorioDiStefano/integration,pasinskim/integration,GregorioDiStefano/integration,pasinskim/integration