t5536: new test of refspec conflicts when fetching

Add some tests that "git fetch" handles refspec conflicts (i.e., when
the same local reference should be updated from two different remote
references) correctly.

There is a small bug when updating references opportunistically,
namely that an explicit user wish like

git fetch origin \
refs/heads/branch1:refs/remotes/origin/branch2 \
refs/heads/branch2:refs/remotes/origin/branch1

should override a configured refspec like

+refs/heads/*:refs/remotes/origin/*

The current code incorrectly treats this as a fatal error.

In a few commits we will improve the error messages for refspec
conflicts in general and also turn this buggy fatal error into a
warning.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>