ENH: Better completion for custom add-ons via new listaddons command.

The todo_completion had the problem that it didn't consider the todo.cfg itself for the location of TODO_ACTIONS_DIR, it just tried the default location or a globally exported config value. With the injection of custom configuration now in place, we can actually delegate the listing to todo.sh itself.
The added built-in "listaddons" command is used for that; it may also be helpful for troubleshooting or to find out about available add-ons. (But the help / shorthelp commands offer more information.)

Additionally, completion is now more precise; only executable actions and no subdirs are listed now; this is also covered by the enhanced test.

The last bonus: The custom add-on actions are now only determined when there's actually completion of commands.

tests/t6050-completion-addons.sh

@@ -7,20 +7,68 @@ This test checks todo_completion of custom actions in .todo.actions.d
 '
 . ./test-lib.sh
 
-readonly ACTIONS='add a addto addm append app archive command del rm depri dp do help list ls listall lsa listcon lsc listfile lf listpri lsp listproj lsprj move mv prepend prep pri p replace report shorthelp'
+readonly ACTIONS='add a addto addm append app archive command del rm depri dp do help list ls listaddons listall lsa listcon lsc listfile lf listpri lsp listproj lsprj move mv prepend prep pri p replace report shorthelp'
 readonly OPTIONS='-@ -@@ -+ -++ -d -f -h -p -P -PP -a -n -t -v -vv -V -x'
 
 readonly ADDONS='bar baz foobar'
-mkdir "$HOME/.todo.actions.d"
-for addon in $ADDONS
-do
-    > "$HOME/.todo.actions.d/$addon"
-    chmod +x "$HOME/.todo.actions.d/$addon"
-done
+makeCustomActions()
+{
+    set -e
+    mkdir "${1:?}"
+    for addon in $ADDONS
+    do
+        addonFile="${1}/$addon"
+        > "$addonFile"
+        chmod +x "$addonFile"
+    done
+
+    # Also create a subdirectory, to test that it is skipped.
+    mkdir "${1}/subdir"
+
+    # Also create a non-executable file, to test that it is skipped.
+    datafile="${1:?}/datafile"
+    > "$datafile"
+    chmod -x "$datafile"
+    [ -x "$datafile" ] && rm "$datafile"    # Some file systems may always make files executable; then, skip this check.
+
+    set +e
+}
+removeCustomActions()
+{
+    set -e
+    rmdir "${1}/subdir"
+    rm "${1:?}/"*
+    rmdir "$1"
+    set +e
+}
+
+#
+# Test resolution of the default TODO_ACTIONS_DIR.
+#
+makeCustomActions "$HOME/.todo.actions.d"
 test_todo_completion 'all arguments' 'todo.sh ' "$ACTIONS $ADDONS $OPTIONS"
 test_todo_completion 'all arguments after option' 'todo.sh -a ' "$ACTIONS $ADDONS $OPTIONS"
 test_todo_completion 'all arguments beginning with b' 'todo.sh b' 'bar baz'
 test_todo_completion 'all arguments beginning with f after options' 'todo.sh -a -v f' 'foobar'
 test_todo_completion 'nothing after addon action' 'todo.sh foobar ' ''
+removeCustomActions "$HOME/.todo.actions.d"
+
+#
+# Test resolution of an alternative TODO_ACTIONS_DIR.
+#
+mkdir  "$HOME/.todo"
+makeCustomActions "$HOME/.todo/actions"
+test_todo_completion 'all arguments with actions from .todo/actions/' 'todo.sh ' "$ACTIONS $ADDONS $OPTIONS"
+removeCustomActions "$HOME/.todo/actions"
+
+#
+# Test resolution of a configured TODO_ACTIONS_DIR.
+#
+makeCustomActions "$HOME/addons"
+cat >> todo.cfg <<'EOF'
+export TODO_ACTIONS_DIR="$HOME/addons"
+EOF
+test_todo_completion 'all arguments with actions from addons/' 'todo.sh ' "$ACTIONS $ADDONS $OPTIONS"
+removeCustomActions "$HOME/addons"
 
 test_done