How to make MSysGit work with non-default key file names?

In order to use multiple keys for multiple GitHub accounts (after this how-to) I want first to get my local system working with key files that are named other than the default id_rsa.

I created a key pair id_rsa_user & id_rsa_user.pub and added the private key part to the SSH Authentification agent and the public key part to GitHub ("Generating SSH Keys").

Then I created the file ~/.ssh/config with this content:

#user account
Host github.com-user
    HostName github.com
    User git
    IdentityFile ~/.ssh/id_rsa_user

Finally I changed the remote URL of the project from git@github.com:user/project.git to git@github.com-user:user/project.git (s. the comments to this how-to).

Now it's working on my VirtualBox-VM (Debian) and in TortoiseGit. But MSysGit throws an error:

(trying to push git@github.com:user/project.git)

***@**** /path/to/to/project (master)
$ git push
fatal: The remote end hung up unexpectedly

(trying to clone git@github.com-user:user/project.git)

***@**** /path/to/to/project (master)
$ git clone git@github.com-user:user/project.git .
Cloning into '.'...
fatal: The remote end hung up unexpectedly

(trying to clone git@github.com:user/project.git ):

***@**** /path/to/to/project (master)
$ git clone git@github.com:user/project.git .
Cloning into '.'...

enter image description here

fatal: The remote end hung up unexpectedly

Does someone have any idea, how to resolve this issue?

Thx

EDIT:

Addition information -- ssh -v git@github.com-user output:

***@**** ~/Desktop/test
$ ssh -v git@github.com-user
OpenSSH_4.6p1, OpenSSL 0.9.8e 23 Feb 2007
debug1: Reading configuration data /c/Users/avex/.ssh/config
debug1: Applying options for github.com-user
debug1: Connecting to github.com [207.97.227.239] port 22.
debug1: Connection established.
debug1: identity file /c/Users/avex/.ssh/id_rsa_user type 1
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.5p1 Debian-6+squeeze1+github8
debug1: match: OpenSSH_5.5p1 Debian-6+squeeze1+github8 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_4.6
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-cbc hmac-md5 none
debug1: kex: client->server aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host 'github.com' is known and matches the RSA host key.
debug1: Found key in /c/Users/avex/.ssh/known_hosts:1
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /c/Users/avex/.ssh/id_rsa_user
debug1: Server accepts key: pkalg ssh-rsa blen 277
debug1: Authentication succeeded (publickey).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
debug1: Remote: Forced command: gerve user 8d:25:62:90:20:4e:89:99:85:42:2c:d6:0a:03:66:dc
debug1: Remote: Port forwarding disabled.
debug1: Remote: X11 forwarding disabled.
debug1: Remote: Agent forwarding disabled.
debug1: Remote: Pty allocation disabled.
debug1: Remote: Forced command: gerve user 8d:25:62:90:20:4e:89:99:85:42:2c:d6:0a:03:66:dc
debug1: Remote: Port forwarding disabled.
debug1: Remote: X11 forwarding disabled.
debug1: Remote: Agent forwarding disabled.
debug1: Remote: Pty allocation disabled.
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
Hi user! You've successfully authenticated, but GitHub does not provide shell access.
debug1: channel 0: free: client-session, nchannels 1
Connection to github.com closed.
debug1: Transferred: stdin 0, stdout 0, stderr 34 bytes in 0.6 seconds
debug1: Bytes per second: stdin 0.0, stdout 0.0, stderr 61.2
debug1: Exit status 1

Solution

Check your environment variables for GIT_SSH. If it is set, then unset it to stop using PuTTY for your SSH client. You're trying to use openssh config options, so you want to use the openssh included with msysgit.