create_from_github(): ensure forked repos are current with upstream/master

[ijlyttle]

Follow-on from #685.

If I create_from_github() on someone else's repository for which I already have a GitHub fork (e.g. r-lib/usethis), the copy that gets cloned to my computer is my master branch, not the upstream master branch. I will probably want to start my work from the upstream master, rather than my likely-stale master. I know that this is easily fixed with pr_pull_upstream(), but I wonder if this might be a useful default behavior for create_from_github().

It would seem to be a matter of adding pr_pull_upstream() to the end of:

usethis/R/create.R

Lines 198 to 202 in 2749025

	if (fork) {
	r <- git2r::repository(proj_get())
	ui_done("Adding {ui_value('upstream')} remote: {ui_value(upstream_url)}")
	git2r::remote_add(r, "upstream", upstream_url)
	}

I'm assuming that the feedback (ui_done()) from pr_pull_upstream() takes care of having to add something here ourselves.

I'll start a PR for this, subject to your guidance.