builtin rebase: only store fully-qualified refs in `options.head_name`

builtin rebase: only store fully-qualified refs in `options.head_name`

When running a rebase on a detached HEAD, we currently store the string
"detached HEAD" in options.head_name. That is a faithful translation of
the shell script version, and we still kind of need it for the purposes of
the scripted backends.

It is poor style for C, though, where we would really only want a valid,
fully-qualified ref name as value, and NULL for detached HEADs, using
"detached HEAD" for display only. Make it so.

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