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.

1 files changed, 3 insertions, 1 deletions

diff --git a/kern.1 b/kern.1
index ac3e4e2..77a3a8b 100644
--- a/kern.1
+++ b/kern.1
@@ -97,7 +97,9 @@ If no template is given, this command uses the output of
 as the template name.
 .It Sy diff Op Ar config
 Uses
-.Xr nvim 1
+.Xr git-diff 1
+if available, otherwise
+.Xr diff 1
 to display the differences between the generated kernel configuration and
 .Em config .
 .Pp