Merge branch 'tc/commit-abbrev-fix'

* tc/commit-abbrev-fix:
  commit::print_summary(): don't use format_commit_message()
  t7502-commit: add summary output tests for empty and merge commits
  t7502-commit: add tests for summary output

diff --cc builtin/commit.c
Simple merge

diff --cc t/t7502-commit.sh
Simple merge