Do bom manual de git(1)
, encontramos
GIT_SSH, GIT_SSH_COMMAND
If either of these environment variables is set then git fetch and
git push will use the specified command instead of ssh when they
need to connect to a remote system. The command will be given
exactly two or four arguments: the username@host (or just host)
from the URL and the shell command to execute on that remote
system, optionally preceded by -p (literally) and the port from the
URL when it specifies something other than the default SSH port.
$GIT_SSH_COMMAND takes precedence over $GIT_SSH, and is interpreted
by the shell, which allows additional arguments to be included.
$GIT_SSH on the other hand must be just the path to a program
(which can be a wrapper shell script, if additional arguments are
needed).
Portanto, a interpretação mais simples via shell é
GIT_SSH_COMMAND="ssh -f" git push ...
ou um wrapper completo via GIT_SSH
.