mx: check existing sessons exactly

From the tmux man page:

  When looking for the session name, tmux initially searches for an
  exact match; if none is found, the session names are checked for any
  for which target-session is a prefix or for which it matches as an
  fnmatch(3) pattern.

This means that the 'tmux has-session' command will return true even if
an exact match wasn't found. This is often useful, but not in this case
since an exact match is wanted.