Merge branch 'jk/read-commit-buffer-data-after-free'

Clarify the ownership rule for commit->buffer field, which some
callers incorrectly accessed without making sure it is populated.

* jk/read-commit-buffer-data-after-free:
  logmsg_reencode: lazily load missing commit buffers
  logmsg_reencode: never return NULL
  commit: drop useless xstrdup of commit message

diff --cc builtin/commit.c
Simple merge

diff --cc commit.h
Simple merge