Add git2r dependency

[mbaynton]

Duplicate of @KimBaldry's #345 but with new CI merged in

[mbaynton]

Hey @KimBaldry it looks like the tests don't quite pass currently. Would you still make use of this on shinyapps.io? If so one way you can test this locally is to run

make test-focal-git2r

I get this failure:

> install.packages("git2r")
now dyn.load("/opt/R/4.2.2/lib/R/library/tools/libs/tools.so") ...
system (cmd0): /opt/R/4.2.2/lib/R/bin/R CMD INSTALL
Error in contrib.url(repos, type) : 
  trying to use CRAN without setting a mirror
Calls: install.packages -> startsWith -> contrib.url
Execution halted

You might try install.packages("git2r", repos="https://cran.rstudio.com")