Git: SSH Key Exchange Improvements

[pjbgf]

Improvements on Git SSH Key Exchange:

• Add curve25519-sha256 and diffie-hellman-group14-sha256 to pkg/ssh
  • ssh: Update PreferredKexAlgos based on golang.org/x/crypto/ssh  pkg#268
• Align supported/preferred algorithms across go-git and libgit2 Managed transport.
  • Add Git test coverage for supported algorithms source-controller#708

Related Issues:

• Weak support for Diffie Hellman groups for SSH KEX #1320