connect: advertized capability is not a ref

When cloning an empty repository served by standard git, "git clone" produces
the following reassuring message:

	$ git clone git://localhost/tmp/empty
	Cloning into 'empty'...
	warning: You appear to have cloned an empty repository.
	Checking connectivity... done.

Meanwhile when cloning an empty repository served by JGit, the output is more
haphazard:

	$ git clone git://localhost/tmp/empty
	Cloning into 'empty'...
	Checking connectivity... done.
	warning: remote HEAD refers to nonexistent ref, unable to checkout.

This is a common command to run immediately after creating a remote repository
as preparation for adding content to populate it and pushing. The warning is
confusing and needlessly worrying.

The cause is that, since v3.1.0.201309270735-rc1~22 (Advertise capabilities
with no refs in upload service., 2013-08-08), JGit's ref advertisement includes
a ref named capabilities^{} to advertise its capabilities on, while git's ref
advertisement is empty in this case. This allows the client to learn about the
server's capabilities and is needed, for example, for fetch-by-sha1 to work
when no refs are advertised.

This also affects "ls-remote". For example, against an empty repository served
by JGit:

	$ git ls-remote git://localhost/tmp/empty
	0000000000000000000000000000000000000000        capabilities^{}

Git advertises the same capabilities^{} ref in its ref advertisement for push
but since it never did so for fetch, the client didn't need to handle this
case.  Handle it.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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