A Jenkins Freestyle job using a git plugin URL 'hostname:path' (the alternate ssh URL syntax) will fail to clone a private repository even when a credential is associated with the repository in the job definition.

The git plugin or git client plugin seems to ignore credentials defined for ssh alternate URL's which do not include a username component.

Steps to duplicate the problem

1. Define an ssh private key credential for the username 'git' to a git provider (I used Assembla)
2. Define a Freestyle job that clones from 'hostname:path'. I used git.assembla.com:git-plugin.bin.git
3. Assign the ssh private key credential to that git repository in the Freestyle job
4. Build the job
5. Expected: Successful build Actual: Failed build

If the URL is changed from git.assembla.com:git-plugin.bin.git to git@git.assembla.com:git-plugin.bin.git, the clone succeeds.

If the URL is changed from git.assembla.com:git-plugin.bin.git to ssh://git.assembla/com:git-plugin.git, the clone succeeds.

If the URL is changed from git.assembla.com:git-plugin.bin.git to ssh://git@git.assembla/com:git-plugin.git, the clone succeeds.