t5800: document some non-functional parts of remote helpers

t5800: document some non-functional parts of remote helpers

These are all things one might expect to work in a helper
that is capable of handling multiple branches (which our
testgit helper in theory should be able to do, as it is
backed by git). All of these bugs are specific to the
import/export codepaths, so they don't affect helpers like
git-remote-curl that use fetch/push commands.

The first and fourth tests are about fetching and pushing
new refs, and demonstrate bugs in the git_remote_helpers
library (so they would be most likely to impact helpers for
other VCSs which import/export git).

The second test is about importing multiple refs; it
demonstrates a bug in git-remote-testgit, which is mostly
for exercising the test code. Therefore it probably doesn't
affect anyone in practice.

The third test demonstrates a bug in git's side of the
helper code when the upstream has added refs that we do not
have locally. This could impact git users who use remote
helpers to access foreign VCSs.

All of those bugs have fixes later in this series.

The fifth test is the most complex, and does not have a fix
in this series. It tests pushing a ref via the export
mechanism to a new name on the remote side (i.e.,
"git push $remote old:new").

The problem is that we push all of the work of generating
the export stream onto fast-export, but we have no way of
communicating to fast-export that this name mapping is
happening. So we tell fast-export to generate a stream with
the commits for "old", but we can't tell it to label them
all as "new".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>