The git ssh command is something very useful, if you - for example -
used to connect to a server via ssh with different identities.
It can be configured with git-config utility

git config core.sshCommand ssh -i <IdentityFile> -l <login>

or it can be given as environment variable given on the command line as

GIT_SSH_COMMAND='ssh -i <IdentityFile> -l <login>' \
git clone ssh://<host>:/home/<login>/<repo.git>

The environment variable has a bigger priority than the git setting.