Remove --show-stash workaround once Git 2.35 is more wide-spread

[woefe]

#40 introduced an optimization for Git 2.35.0 or newer. Once 2.35 is more wide-spread the old workaround should be removed and the code cleaned up.

In 2027, when Ubuntu 22.04 LTS becomes ESM, might be a good time to remove it.

• Require Git 2.35.0
• Remove --show-stash workaround
• Replace GIT_OPTIONAL_LOCKS=0 with --no-optional-locks