Fail early if user tries to use SSH but SSH is not enabled

[jdblischak]

Git allows users to authenticate via a traditional username and password (HTTPS) or SSH keys. The workflowr package also supports SSH keys via git2r/libgit2, but only if it is properly enabled during installation of git2r/libgit2. Instead of waiting for a failure, it would be better if workflowr could catch the error early if it is obvious it will fail.

The protocol that the user is using for authentication is determined for wflow_git_push() in the code below:

https://github.com/jdblischak/workflowr/blob/2bfc2199b949ba608691aff9dfd9d252582f69cf/R/wflow_git_push.R#L140-L149

If protocol == "ssh" and git2r::libgit2_features() returns FALSE, the function throw an error that SSH authentication is not supported, and advise the user to re-install git2r to enable this.

If you'd like to work on this, please volunteer below. Thanks in advance!

Steps:

• Fork and clone workflowr repository
• Checkout the dev branch with "git checkout dev"
• Open the file R/wflow_git_push.R
• After the protocol (HTTPS or SSH) has been determined (lines 140-149), use the git2r function git2r::libgit2_features() to check if SSH is supported. If the user is trying to use SSH when it isn't supported by the current installation, throw an informative error message with stop().
• Repeat the previous step for wflow_git_pull()
• Push to GitHub and submit Pull Request