fix(update-alternatives): fix the "--help" parsing

The format of `update-alternatives --help` and `alternative --help` is
not in the form expected by `_comp_compgen_help`.  This adjusts the
output of `"$1" --help` before passing the output to
`_comp_compgen_help`.

completions/update-alternatives

@@ -83,7 +83,12 @@ _comp_cmd_update_alternatives()
             _comp_cmd_update_alternatives__installed
             ;;
         *)
-            _comp_compgen_help
+            _comp_compgen_help - <<<"$(LANG=C "$1" --help 2>&1 | command sed '
+                /usage:/,/^[[:space:]]*$/{
+                  s/^\([[:space:]]*usage:\)\{0,1\}[[:space:]]*[^[:space:]]*alternatives /  /
+                  s/^[[:space:]]*\[\(-.*\)\]/  \1/
+                }
+                /common options:/,$s/ --/\n --/g')"
             ;;
     esac
 } &&

test/t/test_update_alternatives.py

@@ -6,3 +6,7 @@ class TestUpdateAlternatives:
     @pytest.mark.complete("update-alternatives --", require_cmd=True)
     def test_1(self, completion):
         assert completion
+
+    @pytest.mark.complete("update-alternatives --", require_cmd=True)
+    def test_2(self, bash, completion):
+        assert all(x in completion for x in ["--list", "--remove"])