Re: [PATCH v1 2/4] fsmonitor: update GIT_TEST_FSMONITOR support

Ben Peart [off-list ref] writes:

+if test -n "$GIT_FSMONITOR_TEST"
+then
+	if test -n "$GIT_TEST_FSMONITOR"
+	then
+		echo "warning: the GIT_FSMONITOR_TEST variable has been renamed to GIT_TEST_FSMONITOR"
+	else
+		echo "error: the GIT_FSMONITOR_TEST variable has been renamed to GIT_TEST_FSMONITOR"
+		exit 1
+	fi
+fi

I would have expected that, because we are now doing multiple pairs
of variables in a single series, we would add a helper function that
can be called like so:

	check_var_migration GIT_FSMONITOR_TEST GIT_TEST_FSMONITOR

in the earliest step.  Perhaps something like this.

check_var_migration () {
	old_name=$1 new_name=$2
	eval "old_isset=\${${old_name}:+isset}"
	eval "new_isset=\${${new_name}:+isset}"
	case "$old_isset,$new_isset" in
	isset,)
		echo >&2 "error: $old_name is now $new_name"
		exit 1 ;;
	isset,isset)
		# enable this, once $old_name no longer is valid anywhere
		# echo >&2 "warning: $old_name is now $new_name"
		# echo >&2 "hint: remove $old_name"
		;;
	esac
}