Convert "pop" to the lib infrastructure

Convert "pop" to the lib infrastructure

The test files have to be adapted because of a slightly different way of
recording a "push" operation with the new infrastructure. The original
code was recording two transactions for a "push <patch>" - one for
reordering the unapplied patches and another for pushing the patch. One
would need two "undo" operations to restore the state prior to a "push".
The new infrastructure only records one transaction for "push" and only
one "undo" is necessary.

Since a "pop <middle patch>" operation involves popping the patches and
pushing one, with the above changes in the "push" implementation it
means that the patch order the test scripts expect is slightly
different.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>
Acked-by: Karl Hasselström <kha@treskal.com>