scripts/setlocalversion: factor out 12-chars hash construction

Both of if and else parts append exactly 12 hex chars, but in
different ways.

Factor out the else part because we need to support it without relying
on git-describe. Remove the --abbrev=12 option since we do not use the
hash from git-describe anyway.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Nico Schottelius <nico-linuxsetlocalversion@schottelius.org>