[vim] See the last line of "fzf --version" output

The output may contain some unexpected warning messages from the shell
if it's not properly configured. While such extra messages should be
properly addressed by the user, we can ignore them by checking the
last line of the output instead of the first line.

Related: junegunn/fzf.vim@bd3a021

plugin/fzf.vim

@@ -189,7 +189,7 @@ function! fzf#exec(...)
     if v:shell_error || empty(output)
       throw printf('Failed to run "%s": %s', command, output)
     endif
-    let fzf_version = matchstr(output[0], '[0-9.]\+')
+    let fzf_version = matchstr(output[-1], '[0-9.]\+')
     if s:version_requirement(fzf_version, a:1)
       let s:checked[a:1] = 1
       return s:exec