submodule: look up remotes by URL first

The get_default_remote_submodule() function performs a lookup to find
the appropriate remote to use within a submodule. The function first
checks to see if it can find the remote for the current branch. If this
fails, it then checks to see if there is exactly one remote. It will use
this, before finally falling back to "origin" as the default.

If a user happens to rename their default remote from origin, either
manually or by setting something like clone.defaultRemoteName, this
fallback will not work.

In such cases, the submodule logic will try to use a non-existent
remote. This usually manifests as a failure to trigger the submodule
update.

The parent project already knows and stores the submodule URL in either
.gitmodules or its .git/config.

Add a new repo_remote_from_url() helper which will iterate over all the
remotes in a repository and return the first remote which has a matching
URL.

Refactor get_default_remote_submodule to find the submodule and get its
URL. If a valid URL exists, first try to obtain a remote using the new
repo_remote_from_url(). Fall back to the repo_default_remote()
otherwise.

The fallback logic is kept in case for some reason the user has manually
changed the URL within the submodule. Additionally, we still try to use
a remote rather than directly passing the URL in the
fetch_in_submodule() logic. This ensures that an update will properly
update the remote refs within the submodule as expected, rather than
just fetching into FETCH_HEAD.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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