add a check to dired-subtree--readin, if a listing is already indented

[flokoenig]

Hi,

this is a patch for my problem in #229
Turns out that it isn't a tramp problem. It's a ls --dired problem.

Example listing:

me@mine:~# ls -l
insgesamt 4
drwxr-xr-x 2 root root 4096 13. Apr 18:15 test
-rw-r--r-- 1 root root    0 13. Apr 18:15 test.txt

vs

me@mine:~# ls -l --dired
  insgesamt 4
  drwxr-xr-x 2 root root 4096 13. Apr 18:15 test
  -rw-r--r-- 1 root root    0 13. Apr 18:15 test.txt
//DIRED// 58 62 107 115
//DIRED-OPTIONS// --quoting-style=shell-escape

If --dired is one of the listing switches, the listing will be indented by two whitespaces, insert-directory will clean up the DIRED lines but not the indentation, and then dired-subtree--readin prepends two additional whitespaces. Due to these additional whitespaces, the directories in the subdirectory will not be identified as such and further levels of subdirectories will not work. This is patched here: if two whitespaces are detected in the beginning of the listing, no further whitespaces will be added.

NB: Since tramp for example will use --dired if it is available, a check if --dired is in dired-listing-switches wouldn't suffice, so we simply check for two whitespaces at the beginning of the listing to decide if we still have to prepend.