point pull requesters to GitGitGadget

In the contributing guide and PR template seen by people who open pull
requests on GitHub, we mention the submitGit tool, which gives an
alternative to figuring out the mailing list. These days we also have
the similar GitGitGadget tool, and we should make it clear that this
is also an option.

We could continue to mention _both_ tools, but it's probably better to
pick one in order to avoid overwhelming the user with choice. After all,
one of the purposes here is to reduce friction for first-time or
infrequent contributors. And there are a few reasons to prefer GGG:

  1. submitGit seems to still have a few rough edges. E.g., it doesn't
     munge timestamps to help threaded mail readers handled out-of-order
     delivery.

  2. Subjectively, GGG seems to be more commonly used on the list these
     days, especially by list regulars.

  3. GGG seems to be under more active development (likely related to
     point 2).

So let's actually swap out submitGit for GGG. While we're there, let's
put another link to the GGG page in the PR template, because that's
where users who are learning about it for the first time will want to go
to read more.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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