ENH: Add file completion for addto and listfile.

This enhancement to todo_completion requires a small enhancement to the listfile action: When no SRC is specified, the list of text files in the todo.txt directory is printed. This is probably also useful on its own, and better than the original behavior of printing "TODO: File  does not exist."

Note: I intentionally omitted bullet-proof error handling ($TODO_DIR non-existing or no text files contained), to avoid over-complicating this.

todo.sh

@@ -57,7 +57,7 @@ shorthelp()
 		    list|ls [TERM...]
 		    listall|lsa [TERM...]
 		    listcon|lsc
-		    listfile|lf SRC [TERM...]
+		    listfile|lf [SRC [TERM...]]
 		    listpri|lsp [PRIORITY] [TERM...]
 		    listproj|lsprj [TERM...]
 		    move|mv ITEM# DEST [SRC]
@@ -211,11 +211,13 @@ help()
 		    lsc
 		      Lists all the task contexts that start with the @ sign in todo.txt.
 
-		    listfile SRC [TERM...]
-		    lf SRC [TERM...]
+		    listfile [SRC [TERM...]]
+		    lf [SRC [TERM...]]
 		      Displays all the lines in SRC file located in the todo.txt directory,
 		      sorted by priority with line  numbers.  If TERM specified, lists
 		      all lines that contain TERM in SRC file.
+		      Without any arguments, the names of all text files in the todo.txt
+		      directory are listed.
 
 		    listpri [PRIORITY] [TERM...]
 		    lsp [PRIORITY] [TERM...]
@@ -1049,10 +1051,15 @@ case $action in
 
 "listfile" | "lf" )
     shift  ## Was listfile, next $1 is file name
-    FILE="$1"
-    shift  ## Was filename; next $1 is first search term
+    if [ $# -eq 0 ]; then
+        [ $TODOTXT_VERBOSE -gt 0 ] && echo "Files in the todo.txt directory:"
+        cd "$TODO_DIR" && ls -1 *.txt
+    else
+        FILE="$1"
+        shift  ## Was filename; next $1 is first search term
 
-    _list "$FILE" "$@"
+        _list "$FILE" "$@"
+    fi
     ;;
 
 "listcon" | "lsc" )