push: make upstream, simple work with pushdefault

rr/triangle (4d35924, 2013-04-07) introduced support for triangular
workflows, but did not think through the effect of the new configuration
variables in the upstream and simple modes.  When remote.pushdefault or
branch.<name>.pushremote is set, and push.default is set to upstream or
simple, this happens (master@{u} != origin):

  $ git push
  fatal: You are pushing to remote 'origin', which is not the upstream of
  your current branch 'master', without telling me what to push
  to update which remote branch.

The configuration variables work as expected with push.default = current
and matching, which makes the above unexpected and counter-intuitive.
It happens because upstream and simple were designed with central
workflows in mind.  Even when these configuration variables are not set,

  $ git push origin
  fatal: You are pushing to remote 'origin', which is not the upstream of
  your current branch 'master', without telling me what to push
  to update which remote branch.

This artificial limitation imposed on setup_push_upstream() was
introduced by 135dad (push: error out when the "upstream" semantics does
not make sense, 2012-03-30), but has no basis; a push.default value
should only dictate a refspec, and the push destination is orthogonal to
this.  Remove this artificial limitation, fixing the bug.  Only one test
needs to be changed subtly.

Reported-by: Leandro Lucarella <leandro.lucarella@sociomantic.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
t/push-default: generalize test_push_{success, commit}

The setup creates two bare repositories: repo1 and repo2, but
test_push_commit() hard-codes checking in repo1 for the actual output.
Generalize it and its caller, test_push_success(), to optionally accept
a third argument to specify the name of the repository to check for
actual output.  We will use this in the next patch.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
t/push-default: test pushdefault with all modes

Introduce test_pushdefault_with_mode() to test that remote.pushdefault
works with all the push.default modes correctly.  Exercise it with all
four modes to illustrate how triangular workflows work, and to guard
against regressions.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>