Merge branch 'jt/unparse-commit-upon-graft-change'

Updating the graft information invalidates the list of parents of
in-core commit objects that used to be in the graft file.

* jt/unparse-commit-upon-graft-change:
  commit,shallow: unparse commits if grafts changed

diff --cc commit.c
Simple merge

diff --cc shallow.c
Simple merge