Merge branch 'jc/unresolve' into next

Merge branch 'jc/unresolve' into next

* jc/unresolve:
  Add git-unresolve <paths>...
  get_tree_entry(): make it available from tree-walk
  sha1_name.c: no need to include diff.h; tree-walk.h will do.
  sha1_name.c: prepare to make get_tree_entry() reusable from others.
  pre-commit hook: complain about conflict markers.
  git-merge: a bit more readable user guidance.
  diff: move diff.c to diff-lib.c to make room.
  git log: don't do merge diffs by default
  Allow "git repack" users to specify repacking window/depth