Using GIT_SSH_COMMAND in Git for Windows

You don’t have to set an environment variable anymore in Windows.

With git 2.10+ (Q3 2016), you also have the possibility to set a config for GIT_SSH_COMMAND, which is easier than an environment variable (and can be set globally, or locally for a specific repo)

See commit 3c8ede3 (26 Jun 2016) by Nguyễn Thái Ngọc Duy (pclouds).
^{(Merged by Junio C Hamano — gitster — in commit dc21164, 19 Jul 2016)}

A new configuration variable core.sshCommand has been added to
specify what value for GIT_SSH_COMMAND to use per repository.

core.sshCommand:

If this variable is set, git fetch and git push will use the specified command instead of ssh when they need to connect to a remote system.
The command is in the same form as the GIT_SSH_COMMAND environment variable and is overridden when the environment variable is set.

It means the git push can be:

cd /path/to/my/repo
git config core.sshCommand 'ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no' 
# later on
git push origin master

---

With Git 2.16 (Q1 2018), you will have a new mechanism to upgrade the wire protocol in place is proposed and demonstrated that it works with the older versions of Git without harming them.

See commit 6464679 (16 Oct 2017), and commit 0cd8328 (26 Sep 2017) by Jonathan Tan (jhowtan).
See commit 94b8ae5, commit 3c88ebd, commit 19113a2, commit 0c2f0d2, commit 2609043, commit aa9bab2, commit dfe422d, commit 373d70e, commit 5d2124b (16 Oct 2017) by Brandon Williams (mbrandonw).
^{(Merged by Junio C Hamano — gitster — in commit 4c6dad0, 06 Dec 2017)}

ssh: introduce a ‘simple‘ ssh variant

When using the ‘ssh‘ transport, the ‘-o‘ option is used to specify an
environment variable which should be set on the remote end.
This allows Git to send additional information when contacting the server,
requesting the use of a different protocol version via the
‘GIT_PROTOCOL‘ environment variable like so: “-o SendEnv=GIT_PROTOCOL“.

Unfortunately not all ssh variants support the sending of environment
variables to the remote end.
To account for this, only use the ‘-o‘ option for ssh variants which are OpenSSH compliant.
This is done by checking that the basename of the ssh command is ‘ssh‘ or the ssh variant is overridden to be ‘ssh‘ (via the ssh.variant config).

Other options like ‘-p‘ and ‘-P‘, which are used to specify a specific
port to use, or ‘-4‘ and ‘-6‘, which are used to indicate that IPV4 or
IPV6 addresses should be used, may also not be supported by all ssh
variants.

Currently if an ssh command’s basename wasn’t ‘plink‘ or
‘tortoiseplink‘, Git assumes that the command is an OpenSSH variant.
Since user configured ssh commands may not be OpenSSH compliant, tighten
this constraint and assume a variant of ‘simple‘ if the basename of the
command doesn’t match the variants known to Git.
The new ssh variant ‘simple‘ will only have the host and command to execute ([username@]host command) passed as parameters to the ssh command.