Using just emaclient as editor if server-name is the default value.

Otherwise there are problem on windows (Closes #497)
There is still a problem with non standard server name on windows, but
this commit fix most common case.
Merge branch 'mainline-maint' into mainline-master
Merge branch 'maint' of github.com:magit/magit into maint
Merge branch 'master' of github.com:magit/magit
Merge branch 'maint'
Merge branch 'master' into next