Drop nvim(1) dependency in diff_kernel

Commit 1b8d91f ("Use configuration file under /boot instead of
/proc/config.gz") enables us to drop neovim as a dependency, since we
can now access all configuration data through actual files.

Use git-diff(1) (which has automatic paging & colours) if available to
show the changes between kernel configurations, else fall back to
diff(1).

Since the exit code of diff utilities is >0 if inputs are different,
ignore the exit code of the diff utility in test_diff() to continue like
normal.

0 files changed, 0 insertions, 0 deletions