Commit 009e73d0 (September 2009) changed the job queue to generate
multiple job serials at once. Ever since it would return one more than
requested.

The “serial” file in the job queue directory is defined to contain the
“last job ID used” (design-2.0). With the change above, the serial file
would always contain the next serial number. The first value returned by
the generating function was the one contained in the file, so during the
switch in 2009 one job may have been overwritten.

This patch changes the code to always return the exact number of
serials, to keep the last used serial on disk and adds an assertion.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>