I was facing two issues when using git from within RStudio on DSX:
Both resulting in the error:
> error: cannot run rpostback-askpass: No such file or directory
> fatal: could not read Username for 'https://github.com': No such device or address**
It seems also much harder to troubleshoot this since the shell is disabled and only the only available workaround is via the R console via system("git ...").