[RFC/PATCH] point pull requesters to Git Git Gadget

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 Git Git Gadget 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>
---
I feel a little bad sending this, because I really value the work that
Roberto has done on submitGit. So just dropping it feels a bit
dismissive. But for the reasons above, it seems like GGG is going to be
the path forward, and it doesn't really make much sense to have two
competing systems unless they have really different feature-sets or
approaches (which I don't think is the case).

So I thought I'd mark this RFC and see what people thought. :)

One thing that I think submitGit can do that GGG cannot (yet), is just take PRs straight on git/git. If we're going to start recommending it, then I think we'd probably want to configure that, since it's one less confusing step for first-timers, who right now might have to go re-make their PR on gitgitgadget/git.

 .github/CONTRIBUTING.md          | 2 +-
 .github/PULL_REQUEST_TEMPLATE.md | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)