Don't throw away already started base on resumed create.

Don't throw away already started base on resumed create.

The problem was that after a failure to create the patch base the
resuming tg create did reset HEAD to the next tree to merge in which is
the right thing for a pristine create.

This addresses Debian Bug #513018 and is based on a patch by the bug
reporter Gábor Braun.  A recipe triggering the bug is part of the report
that can be found at http://bugs.debian.org/513018.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>