t/t5528-push-default: remove redundant test_config lines

The line

  test_config push.default upstream

appears unnecessarily in two tests, as the final test_push_failure sets
push.default before pushing anyway.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
config doc: rewrite push.default section

4d3592 (Merge branch 'rr/triangle', 2013-04-07) introduced support for
triangular workflows in Git, but the push.default values still assume
central workflows.  Rewrite the descriptions of `nothing`, `current`,
`upstream` and `matching` for greater clarity, and explicitly explaining
how they behave in triangular workflows.  Leave `simple` as it is for
the moment, as we plan to change its meaning to accommodate triangular
workflows in a later patch.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
push: change `simple` to accommodate triangular workflows

When remote.pushdefault or branch.<name>.pushremote is set (a triangular
workflow feature), master@{u} != origin, and push.default is set to
`upstream` or `simple`:

  $ 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.

Unfortunately, in the case of `upstream`, the very name indicates that
it is only suitable for use in central workflows; let us not even
attempt to give it a new meaning in triangular workflows, and error out
as usual.  However, the `simple` does not have this problem: it is
poised to be the default for Git 2.0, and we would definitely like it to
do something sensible in triangular workflows.

Decouple `simple` from `upstream` completely, and change it to mean
`current` with a safety feature: a `push` and `pull` should not be
asymmetrical in the special case of central workflows.

Reported-by: Leandro Lucarella <leandro.lucarella@sociomantic.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
push: remove dead code in setup_push_upstream()

Now that simple has been decoupled from upstream in setup_push_simple(),
remove the dead code in setup_push_upstream().

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
t/t5528-push-default: generalize test_push_*

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/t5528-push-default: exercise pushdefault workflows

Introduce test_pushdefault_workflows(), and test that all push.default
modes work with central and triangular workflows as expected.

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