cmd/go: add get -f flag

get -u now checks that remote repo paths match the
ones predicted by the import paths: if you are get -u'ing
rsc.io/pdf, it has to be checked out from the right location.
This is important in case the rsc.io/pdf redirect changes.

In some cases, people have good reasons to use
non-standard remote repos. Add -f flag to allow that.
The f can stand for force or fork, as you see fit.

Fixes issue 8850.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/164120043