Add `:silent` in xolox#misc#os#is_mac() to prevent junk on terminal.

[drmikehenry]

The function xolox#misc#os#is_mac() uses Vim's system() function to invoke uname for platform detection. By default, Vim's system() function sets the terminal to "cooked" mode during execution of the external program. Normally this would not be a problem, but there are some sensitive moments during Vim startup where terminal settings are being negotiated; at these times, changing to "cooked" mode can disrupt this negotiation.

In particular, the vim-session plugin uses this is_mac() function during a VimEnter auto-command. When processing VimEnter, terminals such as Kitty are in the midst of terminal negotiations. The visible effect is that escape sequences such as the below may end up on screen after invoking vim with no arguments:

^[[2;2R^[[3;1R^[[>1;4000;20c^[]10;rgb:dddd/dddd/dddd^[^[]11;rgb:0000/0000/0000^[\

To avoid this, use :silent with system() as recommended in :help system().

This problem can be demonstrated in complete isolation from any plugins by having an empty ~/.vim directory and the below three lines as the sole contents of ~/.vimrc:

augroup Something au VimEnter * call system("true", "") augroup END

Using Kitty, the above will reliable generate the junk characters shown above when invoking vim with no arguments.