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.

2 files changed, 10 insertions, 3 deletions

diff --git a/kern b/kern
index 1f49a67..040d150 100755
--- a/kern
+++ b/kern
@@ -77,7 +77,12 @@ config_kernel() {
 
 diff_kernel() {
 	diff=${1:-$(get_running_config)}
-	nvim -d .config "$diff"
+
+	if command -v git >/dev/null; then
+		git diff --no-index "$diff" .config
+	else
+		diff -u "$diff" .config
+	fi
 }
 
 build_kernel() {
@@ -101,7 +106,7 @@ test_diff() {
 	dt=$(mktemp)
 	sed -nE '/^(# )?CONFIG_.*/p' "$(get_running_config)" > "$dt"
 	if ! sed -nE '/^(# )?CONFIG_.*/p' .config | diff -q "$dt" -; then
-		diff_kernel
+		diff_kernel || true
 		printf "Continue? [y/N] "
 		read -r response
 		if test "$response" != "y"; then
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