Load user completions from $BASH_COMPLETION_USER_DIR/completions

Fall back to XDG_DATA_HOME/bash-completion/completions or
~/.local/share/bash-completion/completions if not set.

Thanks to Raphaël Droz.

bash_completion

@@ -1960,13 +1960,15 @@ complete -F _minimal ''
 _completion_loader()
 {
     # $1=_EmptycmD_ already for empty cmds in bash 4.3, set to it for earlier
-    local cmd="${1:-_EmptycmD_}" compdir=./completions compfile
+    local cmd="${1:-_EmptycmD_}" compdir=./completions compfile dir
     [[ $BASH_SOURCE == */* ]] && compdir="${BASH_SOURCE%/*}/completions"
 
-    for compfile in "${cmd##*/}" "${cmd##*/}".bash _"${cmd##*/}"; do
-        compfile="$compdir/$compfile"
-        # Avoid trying to source dirs; https://bugzilla.redhat.com/903540
-        [[ -f "$compfile" ]] && . "$compfile" &>/dev/null && return 124
+    for dir in ${BASH_COMPLETION_USER_DIR:-${XDG_DATA_HOME:-$HOME/.local/share}/bash-completion}/completions $compdir; do
+        for compfile in "${cmd##*/}" "${cmd##*/}".bash _"${cmd##*/}"; do
+            compfile="$dir/$compfile"
+            # Avoid trying to source dirs; https://bugzilla.redhat.com/903540
+            [[ -f "$compfile" ]] && . "$compfile" &>/dev/null && return 124
+        done
     done
 
     # Need to define *something*, otherwise there will be no completion at all.