I'm attempting to explore the pr_* functions from package usethis following along with the pull request article but I've run into some trouble with credentials. It feels like something simple but I haven't figure it out on either of my computers.
I did some initial set-up, getting my GitHub PAT and such into .Renviron following the usethis setup article. I use SSH transport protocol for Git, which has been set up for a while but I found I needed to copy my public key into my GitHub account again to get things working.
I'm on a Windows 7 machine, so the default NULL credentials for SSH don't work out of the box but git2r::cred_ssh_key() returns the correct information so to fork and clone I do
library(usethis)
use_git_credentials(credentials = git2r::cred_ssh_key())
create_from_github("aosmith16/data-manipulation-workshop")
✔ Creating 'C:/Users/muldoona/Desktop/data-manipulation-workshop/'
✔ Cloning repo from 'git@github.com:aosmith16/data-manipulation-workshop.git' into 'C:/Users/muldoona/Desktop/data-manipulation-workshop'
✔ Setting active project to 'C:/Users/muldoona/Desktop/data-manipulation-workshop'
✔ Opening 'C:/Users/muldoona/Desktop/data-manipulation-workshop/' in new RStudio session
✔ Setting active project to '<no active project>'
So that looks OK to me. I switch over to the new instance of RStudio with that project.
Following along with the article, I now want to use pr_init() to make a new branch. I again set my credentials but this time I get an authentication error when using pr_init() (I get the same error if I don't set credentials):
library(usethis)
use_git_credentials(credentials = git2r::cred_ssh_key())
pr_init(branch = "testbranch")
✔ Checking that local branch 'master' has the changes in 'origin/master'
Error in git2r::fetch(repo, name = remref_remote(remref), refspec = branch, :
Error in 'git2r_remote_fetch': error authenticating: failed connecting agent
I can reproduce the error if I use git2r::fetch() with no credentials set. Like
git2r::fetch(".", name = "origin")
Error in git2r::fetch(".", name = "origin") :
Error in 'git2r_remote_fetch': error authenticating: failed connecting agent
If I manually set credentials in git2r::fetch() it doesn't give an error.
git2r::fetch(".", name = "origin", credentials = git2r::cred_ssh_key())
I'm playing around with one of my own repositories, but the same thing happens with the rladies/praise repo from the pull request article.
I've got the latest development versions of usethis and git2r, I've searched error messages and such and have now run out of ideas. 
Can anyone see what I'm missing here?