magit-with-emacsclient: use --socket-name even with default server-name

When actually using a socket for communication, then always pass the
`--socket-name' argument to the Emacsclient.  Do so even when neither
the `server-name' nor `server-socket-dir' have been set to something
different from the default values.

Just because a user did not configure Emacs to use something else,
doesn't necessarily mean that she did not attempt to so, but stopped
after making adjustments to shell init files.  I.e. there are users
with incomplete configurations, who gave up on using the Emacsclient
after having shot themselves in the foot by incompletely configuring
client/server communication.  Magit requires cannot do without the
Emacsclient and so we have to override such mistakes.
Merge branch 'master' into next