RStudio git pane can get out of sync when checking out branch with the `pr_*()` functions

[olivroy]

I'd suggest to add rstudio_git_tickle() in pr_resume()`

https://github.com/r-lib/usethis/blob/da18e11fa311ba7034ab58e86bb7b7e0977d1d26/R/pr.R#L265-L266

Also in pr_finish(), at the end of pr_clean()

I didn't follow the path of usage, but it feels harmless to add it in a couple places.

If you agree, I can open a PR.