Merge branch 'hx/negotiator-non-recursive'

The implementation of the default "negotiator", used to find common
ancestor over the network for object tranfer, used to be recursive;
it was updated to be iterative to conserve stackspace usage.

* hx/negotiator-non-recursive:
  negotiator/skipping: fix some problems in mark_common()
  negotiator/default: avoid stack overflow

diff --cc negotiator/default.c
Simple merge

diff --cc negotiator/skipping.c
Simple merge