Merge branch 'jc/extended-fake-ancestor-for-gitlink'

Merge branch 'jc/extended-fake-ancestor-for-gitlink'

Instead of requiring the full 40-hex object names on the index
line, we can read submodule commit object names from the textual
diff when synthesizing a fake ancestore tree for "git am -3".

* jc/extended-fake-ancestor-for-gitlink:
  apply: verify submodule commit object name better