misc/vim: limit Fmt command to Go buffers.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4548072

misc/vim/ftplugin/go/fmt.vim

diff --git a/misc/vim/ftplugin/go/fmt.vim b/misc/vim/ftplugin/go/fmt.vim
index 18a2156..a299dfc 100644
--- a/misc/vim/ftplugin/go/fmt.vim
+++ b/misc/vim/ftplugin/go/fmt.vim
@@ -13,7 +13,7 @@
 "       replacing the buffer with stderr output.
 "
 
-command! Fmt call s:GoFormat()
+command! -buffer Fmt call s:GoFormat()
 
 function! s:GoFormat()
     let view = winsaveview()