Fix for numbered directories or files

This commit fixes some odd behavior for numbered files and directories. Even if the environment variables is undefined cd'ing into a numbered directory is not possible. 

Below see a short shell transcript showing the issue from a plan bash shell.

    bash-3.2$ pwd
    /tmp/test
    bash-3.2$ git init
    Initialized empty Git repository in /private/tmp/test/.git/
    bash-3.2$ ga
    bash: ga: command not found
    bash-3.2$ mkdir 200
    bash-3.2$ cd 200
    bash-3.2$ pwd
    /tmp/test/200
    bash-3.2$ cd ..
    bash-3.2$ source ~/.scm_breeze/scm_breeze.sh
    bash-3.2$ ga
    Usage: ga <file>  => git add <file>
           ga 1       => git add $e1
           ga 2-4    => git add $e2 $e3 $e4
           ga 2 5-7  => git add $e2 $e5 $e6 $e7

    Note: Deleted files will also be staged using this shortcut.
          To turn off this behaviour, change the 'auto_remove' option.
    bash-3.2$ cd 200
    bash-3.2$ pwd
    /Users/grund

As you can see, I start in a blank folder and initialize a new git repository, then I create a folder called 200 and cd into it. After sourcing the scm_breeze.sh script I can no cd into the directory if it is the only path component.

This patch fixes this behaviour by performing a check if the file or folder exists before applying the substitution for the environment variable for wrapped commands.