Copy patchlogs when cloning a stack or picking a patch.

Copy patchlogs when cloning a stack or picking a patch.

This will allow to keep patch history accross those operations, and
see when they diverged when looking at the respective patchlogs.  With
more work, that will allow to locate the common ancestor when sync'ing
a patch across branches.

The work on "pick" can still be improved: currently the patchlog is
preserved on "pick patch[@branch][//top]", but we could make it work
for //top.old as well (or any rev in the patchlog when we'll be able
to name those).

Patchlog preservation in pick can only work when referencing a patch
by name.  When picking an arbitrary commit we could also improve
things by recording a "pick" operation instead of a "new".

Signed-off-by: Yann Dirson <ydirson@altern.org>