pull: allow "git pull origin $something:$current_branch" into an unborn branch

Some misguided documents floating on the Net suggest this sequence:

mkdir newdir && cd newdir
git init
git remote add origin $url
git pull origin master:master

"git pull" has known about misguided "pull" that lets the underlying fetch
update the current branch for a long time.  It also has known about
"git pull origin master" into a branch yet to be born.

These two workarounds however were not aware of the existence of each
other and did not work well together.  This fixes it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>