Merge branch 'nd/builtin-to-libgit' into next

Merge branch 'nd/builtin-to-libgit' into next

Code cleanups so that libgit.a does not depend on anything in the
builtin/ directory.

Some of the code movement is pretty big, but there doesn't seem to be
any conflicts with topics in flight.

* nd/builtin-to-libgit:
  fetch-pack: move core code to libgit.a
  fetch-pack: remove global (static) configuration variable "args"
  send-pack: move core code to libgit.a
  Move setup_diff_pager to libgit.a
  Move print_commit_list to libgit.a
  Move estimate_bisect_steps to libgit.a
  Move try_merge_command and checkout_fast_forward to libgit.a