submodule--helper: get remote names from any repository

submodule--helper: get remote names from any repository

`get_default_remote()` retrieves the name of a remote by resolving the
refs from of the current repository's ref store.

Thus in order to use it for retrieving the remote name of a submodule,
we have to start a new subprocess which runs from the submodule
directory.

Let's instead introduce a function called `repo_get_default_remote()`
which takes any repository object and retrieves the remote accordingly.

`get_default_remote()` is then defined as a call to
`repo_get_default_remote()` with 'the_repository' passed to it.

Now that we have `repo_get_default_remote()`, we no longer have to start
a subprocess that called `submodule--helper get-default-remote` from
within the submodule directory.

So let's make a function called `get_default_remote_submodule()` which
takes a submodule path, and returns the default remote for that
submodule, all within the same process.

We can now use this function to save an unnecessary subprocess spawn in
`sync_submodule()`, and also in a subsequent patch, which will require
this functionality.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Shourya Shukla <periperidip@gmail.com>
Helped-by: Glen Choo <chooglen@google.com>
Signed-off-by: Atharva Raykar <raykar.ath@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>