feat(kontena): add fallback 3rd-party completion loader

https://github.com/kontena/kontena
https://github.com/kontena/docs/blob/2c87feaf363aa7fd5badc296d5c146dbb89a3c8e/quick-start.md#linux--windows
https://github.com/ohmybash/oh-my-bash/blob/b4a2264496e14f883101ec15e35607b2e7386acc/completions/kontena.completion.sh

completions/Makefile.am

@@ -216,6 +216,7 @@ bashcomp_DATA = 2to3 \
 		kldload \
 		kldunload \
 		koji \
+		_kontena \
 		ktutil \
 		larch \
 		lastlog \

completions/_kontena

@@ -0,0 +1,14 @@
+# 3rd party completion loader for kontena                  -*- shell-script -*-
+#
+# This serves as a fallback in case the completion is not installed otherwise.
+
+# To avoid sourcing an empty string with `. "$(...)"` on failing to obtain the
+# path, we assign the output to a variable `_comp_cmd_kontena__completion_path`
+# and test it before sourcing.  The variable is removed on successful loading
+# but left on a failure for the debugging purpose.
+_comp_cmd_kontena__completion_path=$("$1" whoami --bash-completion-path 2>/dev/null) &&
+    [[ -r $_comp_cmd_kontena__completion_path ]] &&
+    . "$_comp_cmd_kontena__completion_path" &&
+    unset -v _comp_cmd_kontena__completion_path
+
+# ex: filetype=sh

test/fallback/completions/Makefile.am

@@ -15,6 +15,7 @@ EXTRA_DIST = \
 	hwclock \
 	ionice \
 	keyring \
+	kontena \
 	look \
 	mdbook \
 	mock \

test/fallback/completions/kontena

@@ -0,0 +1 @@
+../../../completions/_kontena