t7406: correct test case for submodule-update initial population

t7406: correct test case for submodule-update initial population

There are three issues with the test:

* The syntax of the here-doc was wrong, such that the entire test was
  sucked into the here-doc, which is why the test succeeded.

* The variable $submodulesha1 was not expanded as it was inside a quoted
  here text.  We do not want to quote EOF marker for this.

* The redirection from the git command to the output file for comparison
  was wrong as the -C operator from git doesn't apply to the redirect path.
  Also we're interested in stderr of that command.

Noticed-by: Jan Palus <jan.palus@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>