Set IdentitiesOnly=yes when using key_file (#5682)

Sets the SSH option `IdentitiesOnly=yes` in the SSH wrapper when a
`key_file` is provided to the git module. This option ensures that
the provided key is used. Otherwise, the system's ssh-agent could
provide undesired identities when connecting.

From ssh_config(5):

> Specifies that ssh(1) should only use the authentication identity and
> certificate files explicitly configured in the ssh_config files or
> passed on the ssh(1) command-line, even if ssh-agent(1) or a
> PKCS11Provider offers more identities. The argument to this keyword
> must be “yes” or “no”. This option is intended for situations where
> ssh-agent offers many different identities. The default is “no”.

lib/ansible/modules/source_control/git.py

@@ -320,7 +320,7 @@ fi
 if [ -z "$GIT_KEY" ]; then
     ssh $BASEOPTS "$@"
 else
-    ssh -i "$GIT_KEY" $BASEOPTS "$@"
+    ssh -i "$GIT_KEY" -o IdentitiesOnly=yes $BASEOPTS "$@"
 fi
 """
     fh.write(template)