pw-apply: Support specifying application base

For cross-subsystem changes it's useful to use an upstream tag
as base of applying patches:

          merge commit
         /         |
       /           |
  new patch 2      |
       |           |
  new patch 1      |
       |           |
       |       origin/main
       |           |
       |          ...
       |           |
       |       net merge
       |         / |
       |       /   |
       |     /    ...
       |   /
     vX.Y-rcZ

Add an option (-u) for selecting base where patches are applied.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>