Merge branch 'ds/fix-thin-fix'

"git index-pack --fix-thin" used to abort to prevent a cycle in
delta chains from forming in a corner case even when there is no
such cycle.

* ds/fix-thin-fix:
  index-pack: allow revisiting REF_DELTA chains
  t5309: create failing test for 'git index-pack'
  test-tool: add pack-deltas helper

diff --cc Makefile
Simple merge

diff --cc builtin/index-pack.c
Simple merge