parse-remote: mark all refs not for merge only when fetching more than one

parse-remote: mark all refs not for merge only when fetching more than one

An earlier commit a71fb0a1 implemented much requested safety
valve to refuse "git pull" or "git pull origin" without explicit
refspecs from using the first set of remote refs obtained by
reading .git/remotes/origin file or branch.*.fetch configuration
variables to create a merge.  The argument was that while on a
branch different from the default branch, it is often wrong to
merge the default remote ref suitable for merging into the master.

That is fine as a theory.  But many repositories already in use
by people in the real world do not have any of the per branch
configuration crap.  They did not need it, and they do not need
it now.  Merging with the first remote ref listed was just fine,
because they had only one ref (e.g. 'master' from linux-2.6.git)
anyway.

So this changes the safety valve to be a lot looser.  When "git
fetch" gets only one remote branch, the irritating warning would
not trigger anymore.

I think we could also make the warning trigger when branch.*.merge
is not specified for the current branch, but is for some other
branch.  That is for another commit.

Signed-off-by: Junio C Hamano <junkio@cox.net>