The new Git for Windows places the SSH binary in a usr/bin/ subdirectory instead of the bin/ directory used by prior releases of msysgit. This results in an error:
FATAL: ssh executable not found. The git plugin only supports official git client http://git-scm.com/download/win
java.lang.RuntimeException: ssh executable not found. The git plugin only supports official git client http://git-scm.com/download/win
The 'official' link takes you to a download for this new Git for Windows, so it's not helpful.
A temporary workaround is to create a link to ssh.exe in the $(INSTALLDIR)\bin directory with mklink, e.g.:
mklink "c:\program files\git\bin\ssh.exe" "c:\program files\git\usr\bin\ssh.exe"
Another thing I noticed, not related to the jenkins plugin, is that the new Git for Windows prepends the NetBIOS domain name in a Windows AD environment by default. So existing jobs which don't explicitly set the username will fail, because the default username is now DOMAIN+username.