Use git worktree in deploy_local

[jimhester]

This avoids the need to determine the remote url, as we can reuse the
existing one in the checked out repository.

This also allows us to use deploy_local() in GitHub Actions

[jimhester]

I now export deploy_local() which we didn't previously. I think this is the right thing to do, it is a useful function and we need to call it for GitHub Actions.

The other benefit to this approach using git worktree is that now we can push to the gh-pages branch without having to setup any secrets or add a DEPLOY_PAT in GitHub Actions, which is nice!

An example workflow file using the changes in this PR is at https://github.com/jimhester/r-actions-test/actions/runs/38247939/workflow#L27-L31

[jimhester]

We need to double check that these changes don't break deploy_site_github(), but I believe they will be OK.

[hadley]

Now just needs a news bullet, I think.

[jimhester]

Ok added two news bullets about the new function and deprecating the repo_slug argument.

[Bisaloo]

I may be completely wrong here but shouldn't this be:

github_worktree_add(dest_dir, branch = branch)

?

We're getting an error when trying to deploy from travis with the dev version of pkgdown:

Error in git("worktree", "add", "--track", "-b", branch, dir, paste0("origin/", :

argument "branch" is missing, with no default

Complete log: https://travis-ci.org/rmaia/pavo/jobs/651466810

[jimhester]

You are right @Bisaloo, should be fixed by aceba8b

[sumedh10]

This change now throws the following error when travis tries to deploy on enterprise github

Running git worktree add --track -b gh-pages /tmp/Rtmp8zQQdq/file598365058050
origin/gh-pages
Preparing worktree (new branch 'gh-pages')
error: the requested upstream branch 'origin/gh-pages' does not exist
...
...
...
Error: System command 'git' failed, exit status: 255, stdout & stderr were printed

Btw, gh-pages branch does exist on our repos.