If ssh-agent.exe is running, it prevents upgrades of Git for Windows

[dscho]

As we know that ssh-agent.exe is a daemon, we should simply kill it and move on.

The main problem I had with this is that it appears as if there may be an ssh-add.exe running at the same time, and a second ssh-agent.exe process, both of which are not shown by MSYS2's own ps -W. But the Task Manager sees them and the user can kill them interactively.

Hopefully we can teach the installer to kill silently any of Git's own ssh-agent.exe and/or ssh-add.exe. The chances are good: We already show such processes successfully in the wizard page that prevents the installer from running, after all.

[landstander668]

For what it's worth, ssh-pageant.exe should probably be handled in the same way. It has essentially the same impact and is reported the same way when upgrading, although I wouldn't expect to find the ssh-add.exe and/or second agent processes in that case.