branch.c: Relax unnecessary requirement on upstream's remote ref name

branch.c: Relax unnecessary requirement on upstream's remote ref name

When creating an upstream relationship, we use the configured remotes and
their refspecs to determine the upstream configuration settings
branch.<name>.remote and branch.<name>.merge. However, if the matching
refspec does not have refs/heads/<something> on the remote side, we end
up rejecting the match, and failing the upstream configuration.

It could be argued that when we set up an branch's upstream, we want that
upstream to also be a proper branch in the remote repo. Although this is
typically the common case, there are cases (as demonstrated by the previous
patch in this series) where this requirement prevents a useful upstream
relationship from being formed. Furthermore:

 - We have fundamentally no say in how the remote repo have organized its
   branches. The remote repo may put branches (or branch-like constructs
   that are insteresting for downstreams to track) outside refs/heads/*.

 - The user may intentionally want to track a non-branch from a remote
   repo, by using a branch and configured upstream in the local repo.

Relaxing the checking to only require a matching remote/refspec allows the
testcase introduced in the previous patch to succeed, and has no negative
effect on the rest of the test suite.

This patch fixes a behavior (arguably a regression) first introduced in
41c21f2 (branch.c: Validate tracking branches with refspecs instead of
refs/remotes/*) on 2013-04-21 (released in >= v1.8.3.2).

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