RepoCommand#readFile: Don't call Git#getRepository() in try-with-resource

RepoCommand#readFile: Don't call Git#getRepository() in try-with-resource

Using try-with-resource means that close() will automatically be
called on the Repository object. However, according to the javadoc
of Git#close():

  If the repository was opened by a static factory method in this class,
  then this method calls Repository#close() on the underlying repository
  instance.

This means that Repository#close() is called twice, by Git.close()
and in the outer try-with-resource, leading to a corrupt use count.

Change-Id: I37ba517eb2cc67d1cd36813598772c70208d0bc9
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>