feat(jj): add 3rd party fallback completion loader

https://github.com/martinvonz/jj/blob/main/docs/install-and-setup.md#command-line-completion

Author: scop

completions/Makefile.am

@@ -204,6 +204,7 @@ cross_platform = 2to3 \
 		jarsigner \
 		java \
 		javaws \
+		_jj \
 		jq \
 		jpegoptim \
 		jps \

completions/_jj

@@ -0,0 +1,8 @@
+# 3rd party completion loader for commands emitting        -*- shell-script -*-
+# their completion using "$cmd util completion bash".
+#
+# This serves as a fallback in case the completion is not installed otherwise.
+
+eval -- "$("$1" util completion bash 2>/dev/null)"
+
+# ex: filetype=sh

test/fallback/completions/Makefile.am

@@ -16,6 +16,7 @@ EXTRA_DIST = \
 	hwclock \
 	insmod \
 	ionice \
+	jj \
 	jungle \
 	keyring \
 	kontena \

test/fallback/completions/jj

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