Refactor s:TmuxCommand, add s:GetTmuxCommand

[blueyed]

• use a list instead of single string
• add s:GetTmuxCommand to be used later when not using system() only
• remove :silent when calling tmux from s:TmuxAwareNavigate: it
  should not be necessary and is bad practice to use :silent
  unnecessarily. It was added in b068a04 with no explanation.

[christoomey]

Broadly these changes all look good to me, but unfortunately I'm getting an error when running with this locally:

Error detected while processing function <SNR>151_TmuxWinCmd[2]..<SNR>151_TmuxAwareNavigate[23]..<SNR>151_TmuxCommand:
line    2:
E730: using List as a String

Any thoughts on what the issue might be?

---

Adding a bit more detail, I'm able to move between Vim panes, but hit the above error when attempting to navigate from Vim -> tmux.

[blueyed]

@christoomey
The problem is that Vim's system() does not handle lists for the command/argv. Will fix it.