revision walker: include a detached HEAD in --all

revision walker: include a detached HEAD in --all

When HEAD is detached, --all should list it, too, logically, as a
detached HEAD is by definition a temporary, unnamed branch.

It is especially necessary to list it when garbage collecting, as
the detached HEAD would be trashed.

Noticed by Thomas Rast.

Note that this affects creating bundles with --all; I contend that it
is a good change to add the HEAD, so that cloning from such a bundle
will give you a current branch.  However, I had to fix t5701 as it
assumed that --all does not imply HEAD.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>