git: only attempt ssh-agent authentication once
git: only attempt ssh-agent authentication once

This prevents infinite looping if ssh-agent authentication fails.

closes #2845

rust-lang/cargo,nipunn1313/cargo,AndiDog/cargo,rust-lang/cargo,tshepang/cargo,sbeckeriv/cargo,tshepang/cargo,durka/cargo,AndiDog/cargo,nipunn1313/cargo,tshepang/cargo,eminence/cargo,brson/cargo,vojtechkral/cargo,achanda/cargo,rustyhorde/cargo,vojtechkral/cargo,achanda/cargo,nipunn1313/cargo,nipunn1313/cargo,rust-lang/cargo,brson/cargo,brson/cargo,rustyhorde/cargo,tsurai/cargo,brson/cargo,AndiDog/cargo,tsurai/cargo,tsurai/cargo,vojtechkral/cargo,sbeckeriv/cargo,achanda/cargo,achanda/cargo,sbeckeriv/cargo,durka/cargo,AndiDog/cargo,durka/cargo,tsurai/cargo,durka/cargo,rustyhorde/cargo,vojtechkral/cargo,eminence/cargo,tsurai/cargo,brson/cargo,rustyhorde/cargo,AndiDog/cargo,sbeckeriv/cargo,rust-lang/cargo,tshepang/cargo,eminence/cargo,eminence/cargo