vim: fix go filetype detection
vim: fix go filetype detection

The filetype needs to be set during BufRead in order for the did_filetype() check to prevent the file being detected as a conf file.  One example where this can occur is if a cgo file has a #include at the top of the file.  The # is detected in vim's generic configuration (conf file) toward the bottom of filetype.vim

R=golang-dev, dsymonds
CC=golang-dev
http://codereview.appspot.com/5496066

Committer: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang