I just created a sandbox project on drupal.org. I was getting following error.
Unable to negotiate with 188.8.131.52: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1 fatal: Could not read from remote repository.
How to resolve this: please run following command to overcome this error.
export GIT_SSH_COMMAND='ssh -o KexAlgorithms=+diffie-hellman-group1-sha1'