Generate unique patch names

"stg assimilate" was already making sure that automatically generated
patch names were non-empty and unique, by suffixing them with a dash
and a number if necessary. This patch moves that functionality into
the autogeneration function itself, so that all its callers can
benefit from it (and the user can benefit from uniform behavior from
all stgit commands).

As an added bonus, this permits the removal of a number of checks that
would abort with an error if the automatically generated name was
empty.

Signed-off-by: Karl Hasselström <kha@treskal.com>