Merge branch 'jc/fake-ancestor-with-non-blobs'

Rebasing the history of superproject with change in the submodule
was broken since v1.7.12.

* jc/fake-ancestor-with-non-blobs:
  apply: diagnose incomplete submodule object name better
  apply: simplify build_fake_ancestor()
  git-am: record full index line in the patch used while rebasing

diff --cc builtin/apply.c
Simple merge

diff --cc git-am.sh
Simple merge