clone: use remote branch if it matches default HEAD

clone: use remote branch if it matches default HEAD

Usually clone tries to use the same local HEAD as the remote (unless the
user has given --branch explicitly). Even if the remote HEAD is detached
or unborn, we can detect those situations with modern versions of Git.
If the remote is too old to support the "unborn" extension (or it has
been disabled via config), then we can't know the name of the remote's
unborn HEAD, and we fall back whatever the local default branch name is
configured to be.

But that leads to one weird corner case. It's rare because it needs a
number of factors:

  - the remote has an unborn HEAD

  - the remote is too old to support "unborn", or has disabled it

  - the remote has another branch "foo"

  - the local default branch name is "foo"

In that case you end up with a local clone on an unborn "foo" branch,
disconnected completely from the remote's "foo". This is rare in
practice, but the result is quite confusing.

When choosing "foo", we can double check whether the remote has such a
name, and if so, start our local "foo" at the same spot, rather than
making it unborn.

Note that this causes a test failure in t5605, which is cloning from a
bundle that doesn't contain HEAD (so it behaves like a remote that
doesn't support "unborn"), but has a single "main" branch. That test
expects that we end up in the weird "unborn main" case, where we don't
actually check out the remote branch of the same name. Even though we
have to update the test, this seems like an argument in favor of this
patch: checking out main is what I'd expect from such a bundle.

So this patch updates the test for the new behavior and adds an adjacent
one that checks what the original was going for: if there's no HEAD and
the bundle _doesn't_ have a branch that matches our local default name,
then we end up with nothing checked out.

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