checkout: Use remote refspecs when DWIMming tracking branches

The DWIM mode of checkout allows you to run "git checkout foo" when there
is no existing local ref or path called "foo", and there is exactly _one_
remote with a remote-tracking branch called "foo". Git will automatically
create a new local branch called "foo" using the remote-tracking "foo" as
its starting point and configured upstream.

For example, consider the following unconventional (but perfectly valid)
remote setup:

	[remote "origin"]
		fetch = refs/heads/*:refs/remotes/origin/*
	[remote "frotz"]
		fetch = refs/heads/*:refs/remotes/frotz/nitfol/*

Case 1: Assume both "origin" and "frotz" have remote-tracking branches called
"foo", at "refs/remotes/origin/foo" and "refs/remotes/frotz/nitfol/foo"
respectively. In this case "git checkout foo" should fail, because there is
more than one remote with a "foo" branch.

Case 2: Assume only "frotz" have a remote-tracking branch called "foo". In
this case "git checkout foo" should succeed, and create a local branch "foo"
from "refs/remotes/frotz/nitfol/foo", using remote branch "foo" from "frotz"
as its upstream.

The current code hardcodes the assumption that all remote-tracking branches
must match the "refs/remotes/$remote/*" pattern (which is true for remotes
with "conventional" refspecs, but not true for the "frotz" remote above).
When running "git checkout foo", the current code looks for exactly one ref
matching "refs/remotes/*/foo", hence in the above example, it fails to find
"refs/remotes/frotz/nitfol/foo", which causes it to fail both case #1 and #2.

The better way to handle the above example is to actually study the fetch
refspecs to deduce the candidate remote-tracking branches for "foo"; i.e.
assume "foo" is a remote branch being fetched, and then map "refs/heads/foo"
through the refspecs in order to get the corresponding remote-tracking
branches "refs/remotes/origin/foo" and "refs/remotes/frotz/nitfol/foo".
Finally we check which of these happens to exist in the local repo, and
if there is exactly one, we have an unambiguous match for "git checkout foo",
and may proceed.

This fixes most of the failing tests introduced in the previous patch.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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