A super easy way to grant access to a AWS CodeCommit repository from an EC2 instance hosting e.g. Jenkins is to grant proper IAM Role permissions

{
 "Effect": "Allow",
 "Action": "codecommit:*",
 "Resource": "*"
}

and configure git client to use a helper:

git config --global credential.helper '!aws codecommit credential-helper $@'
git config --global credential.useHttpPath true

Voila!