Issue:
A checkout from the master can not be performed when using a ssh key with a passphrase. It will hang on this step:

> git fetch --tags --progress git@github.com:alexanderrtaylor/support-kb-articles.git +refs/heads/*:refs/remotes/origin/*

And eventually fail.

Workaround:
You can remove the passphrase and it will work normally

Steps to reproduce:
1. Create a SSH key with a specific passphrase like `testing` and add it to Jenkins
2. Add ssh key to github
3. Create a job(freestyle or pipeline the behavior is the same)
4. Use the SSH key connect to the ssh location of the git url
5. Watch as the job fails.

This works normally if I use a shell step to clone from the agent but the git plugin seems to be causing the issue