completion: improve untracked directory filtering for filename completion

Similar to Bash's default filename completion, our git-aware filename
completion stops at directory boundaries, i.e. it doesn't offer the
full 'path/to/file' at first, but only 'path/'.  To achieve that the
completion script runs 'git ls-files' with specific command line
options to get the list of relevant paths under the current directory,
and then processes each path to strip all but the base directory or
filename (see __git_index_files()).

To offer only modified and untracked files for 'git add' the
completion script runs 'git ls-files --exclude-standard --others
--modified'.  This command lists all non-ignored files in untracked
directories, which leads to a noticeable delay caused by the
processing mentioned above if there are a lot of such files
(__git_index_files() specifies '--exclude-standard' internally):

  $ mkdir untracked-dir
  $ for i in {1..10000} ; do >untracked-dir/$i ; done
  $ time __git_index_files "--others --modified"
  untracked-dir

  real	0m0.537s
  user	0m0.452s
  sys	0m0.160s

Eliminate this delay by additionally passing the '--directory
--no-empty-directory' options to 'git ls-files' to show only the
directory name of non-empty untracked directories instead their whole
content:

  $ time __git_index_files "--others --modified --directory --no-empty-directory"
  untracked-dir

  real	0m0.029s
  user	0m0.020s
  sys	0m0.004s

Filename completion for 'git clean' suffers from the same delay, as it
offers untracked files, too.  The fix could be the same, but since it
actually makes sense to 'git clean' empty directories, in this case we
only pass the '--directory' option to 'git ls-files'.

Reported-by: Isaac Levy <ilevy@google.com>
Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -901,7 +901,7 @@ _git_add ()
 	esac
 
 	# XXX should we check for --update and --all options ?
-	__git_complete_index_file "--others --modified"
+	__git_complete_index_file "--others --modified --directory --no-empty-directory"
 }
 
 _git_archive ()
@@ -1063,7 +1063,7 @@ _git_clean ()
 	esac
 
 	# XXX should we check for -x option ?
-	__git_complete_index_file "--others"
+	__git_complete_index_file "--others --directory"
 }
 
 _git_clone ()