Allow for remote names other than origin.

[RomeoV]

We introduce getremotename that tries to check the remote.pushDefault option to determine the remote name, and falls back to origin if it's not set.

Fixes #2464 and perhaps #1916, as well as https://discourse.julialang.org/t/creating-documentation-when-remote-name-is-not-origin/110466.

[mortenpi]

Generally, I think trying to use pushDefault makes sense. But I have a couple of concerns:

1. It will warn when remote.pushDefault is not set. However, it doesn't seem that remote.pushDefault is set in the normal case (e.g. you have just git clone-d a repo). So it seems that 99% of the time this code will warn (you can see that in the CI logs too), which is not great.

2. I also wonder what is the right behavior when you do have origin, but pushDefault points to a different remote? I generally don't set pushDefault, but the use case I can imagine is when I copy a repo that I don't have access to, then fork it on GitHub and set pushDefault to the fork under my user. But if I were to build docs locally, I think I would prefer if it would pick origin over my fork. So maybe it should use origin if available, and if there is not remote called origin, it will pick the second best on based on pushDefault.

   Or maybe first step could be to just pick the only remote that is available, even if it is not called origin? And only use pushDefault if you have multiple remotes, none are called origin, and then use pushDefault to disambiguate?

3. It would be good to add a note somewhere on the docs/src/lib/remote-links.md manual page about this behavior.

4. Ideally there would also be a test. We have some related tests in test/repolinks.jl

[RomeoV]

Thanks for the thoughts. I have changed the workflow to default to origin, then remotes.pushDefault with a warning, and then just "" (which was the behavior before) with another warning.
I also added a related comment to the docs, and wrote some tests.