ls-remote: add support for showing symrefs

ls-remote: add support for showing symrefs

Sometimes it's useful to know the main branch of a git repository
without actually downloading the repository.  This can be done by
looking at the symrefs stored in the remote repository.  Currently git
doesn't provide a simple way to show the symrefs stored on the remote
repository, even though the information is available.  Add a --symref
command line argument to the ls-remote command, which shows the symrefs
in the remote repository.

While there, replace a literal tab in the format string with \t to make
it more obvious to the reader.

Suggested-by: pedro rijo <pedrorijo91@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>