Support Windows supplied OpenSSH with bundled Git

[Okeanos]

Problem description

The Git executable bundled with Sublime Merge is unable to communicate with the Windows supplied OpenSSH binaries.

Preferred solution

Sublime Merge should improve the SSH support on Windows as an alternative to Pageant/Plink and enable the bundled Git executable to talk to the Windows native SSH agent to retrieve keys. This would streamline some workflows and allow for better tool integration. This would also align workflows more closely with the Unix & macOS solutions and improve the user experience by requiring less additional software.

The documentation should also be updated, at least containing the workaround description.

Alternatives

An alternative that does work already is using a system supplied/external Git installation. If correctly configured (see git-for-windows/build-extra/pull/367 for additional details), external Git and Sublime Merge will use Windows supplied OpenSSH.