submodule--helper: fix incorrect newlines in an error message

submodule--helper: fix incorrect newlines in an error message

A refactoring[1] done as part of the recent conversion of
'git submodule add' to builtin, changed the error message
shown when a Git directory already exists locally for a submodule
name. Before the refactoring, the error used to appear like so:

  --- START OF OUTPUT ---
  $ git submodule add ../sub/ subm
  A git directory for 'subm' is found locally with remote(s):
    origin        /me/git-repos-for-test/sub
  If you want to reuse this local git directory instead of cloning again from
    /me/git-repos-for-test/sub
  use the '--force' option. If the local git directory is not the correct repo
  or you are unsure what this means choose another name with the '--name' option.
  ---  END OF OUTPUT  ---

After the refactoring the error started appearing like so:

  --- START OF OUTPUT ---
  $ git submodule add ../sub/ subm
  A git directory for 'subm' is found locally with remote(s):  origin     /me/git-repos-for-test/sub
  fatal: If you want to reuse this local git directory instead of cloning again from
  /me/git-repos-for-test/sub
  use the '--force' option. If the local git directory is not the correct repo
  or if you are unsure what this means, choose another name with the '--name' option.

  ---  END OF OUTPUT  ---

As one could observe the remote information is printed along with the
first line rather than on its own line. Also, there's an additional
newline following output.

Make the error message consistent with the error message that used to be
printed before the refactoring.

This also moves the 'fatal:' prefix that appears in the middle of the
error message to the first line as it would more appropriate to have
it in the first line. The output after the change would look like:

  --- START OF OUTPUT ---
  $ git submodule add ../sub/ subm
  fatal: A git directory for 'subm' is found locally with remote(s):
    origin        /me/git-repos-for-test/sub
  If you want to reuse this local git directory instead of cloning again from
    /me/git-repos-for-test/sub
  use the '--force' option. If the local git directory is not the correct repo
  or you are unsure what this means choose another name with the '--name' option.
  ---  END OF OUTPUT  ---

[1]: https://lore.kernel.org/git/20210710074801.19917-5-raykar.ath@gmail.com/#t

Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>