Allow git-checkout when on a non-existant branch.

Allow git-checkout when on a non-existant branch.

I've seen some users get into situtations where their HEAD
symbolic-ref is pointing at a non-existant ref.  (Sometimes this
happens during clone when the remote repository lacks a 'master'
branch.)  If this happens the user is unable to use git-checkout
to switch branches as there is no prior commit to merge from.

So instead of giving the user low-level errors about how HEAD
can't be resolved and how not a single revision was given change
the type of checkout to be a force and go through with the user's
request anyway.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>