Context:
Recent versions of git (>= v2.10) support setting an SSH command via .gitconfig files. E.g., https://stackoverflow.com/a/38474137/93345 .

This is particularly useful when coupled with another somewhat recent feature of conditional config imports e.g., https://stackoverflow.com/a/36296990/93345 .

Problem:
The setupGitEnv() function in get_git.go specifies a GIT_SSH_COMMAND environment variable, even when an ssh key file is not explicitly set in the git repo URL via the sshkey query parameter.

When there is no query parameter for sshkey, the GIT_SSH_COMMAND environment variable should be left unset. Presently, it ends up being set to GIT_SSH_COMMAND=ssh, which breaks configuration via .gitconfig files.