ENH: TODOTXT_SOURCEVAR enables different source file for listcon.

To be used by todo_completion (to offer candidates from done.txt when there are no matches in todo.txt), and maybe by custom addons.

todo.sh

@@ -136,7 +136,7 @@ help()
 
 	EndOptionsHelp
 
-    [ $TODOTXT_VERBOSE -gt 1 ] && cat <<-EndVerboseHelp
+    [ $TODOTXT_VERBOSE -gt 1 ] && cat <<-'EndVerboseHelp'
 		  Environment variables:
 		    TODOTXT_AUTO_ARCHIVE            is same as option -a (0)/-A (1)
 		    TODOTXT_CFG_FILE=CONFIG_FILE    is same as option -d CONFIG_FILE
@@ -149,6 +149,7 @@ help()
 		    TODOTXT_DEFAULT_ACTION=""       run this when called with no arguments
 		    TODOTXT_SORT_COMMAND="sort ..." customize list output
 		    TODOTXT_FINAL_FILTER="sed ..."  customize list after color, P@+ hiding
+		    TODOTXT_SOURCEVAR=\$DONE_FILE   use another source for listcon
 
 
 	EndVerboseHelp
@@ -1128,7 +1129,9 @@ case $action in
     ;;
 
 "listcon" | "lsc" )
-    grep -o '[^ ]*@[^ ]\+' "$TODO_FILE" | grep '^@' | sort -u
+    FILE=$TODO_FILE
+    [ "$TODOTXT_SOURCEVAR" ] && eval "FILE=$TODOTXT_SOURCEVAR"
+    grep -ho '[^ ]*@[^ ]\+' "${FILE[@]}" | grep '^@' | sort -u
     ;;
 
 "listproj" | "lsprj" )