misc/vim: restore fileencodings.

Currently, ftdetect/gofiletype.vim set fileencodings to open the file as
utf-8 encoding event if the file does not contain multibyte characters.
But fileencodings is global option.

$ vim foo.txt
:set fileencodings
utf-8,ucs-bom,cp932

$ vim foo.go
:set fileencodings
utf-8

This change restore fileencodings before opening the file.
Also added specify fileformats=unix.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5718045

misc/vim/ftdetect/gofiletype.vim

-au BufRead,BufNewFile *.go set filetype=go fileencoding=utf-8 fileencodings=utf-8
+" We take care to preserve the user's fileencodings and fileformats,
+" because those settings are global (not buffer local), yet we want
+" to override them for loading Go files, which are defined to be UTF-8.
+let s:current_fileformats = ''
+let s:current_fileencodings = ''
+
+" define fileencodings to open as utf-8 encoding even if it's ascii.
+function! s:gofiletype_pre()
+  let s:current_fileformats = &g:fileformats
+  let s:current_fileencodings = &g:fileencodings
+  set fileencodings=utf-8 fileformats=unix
+  setlocal filetype=go
+endfunction
+
+" restore fileencodings as others
+function! s:gofiletype_post()
+  let &g:fileformats = s:current_fileformats
+  let &g:fileencodings = s:current_fileencodings
+endfunction
+
+au BufNewFile *.go setlocal filetype=go fileencoding=utf-8 fileformat=unix
+au BufRead *.go call s:gofiletype_pre()
+au BufReadPost *.go call s:gofiletype_post()