Merge branch 'sk/test-commit-body-helper' into seen

A new test helper commit_body() has been introduced to print the
message body of a commit, and various tests have been updated to use
it instead of spelling out the command pipeline manually and losing
the exit status of the 'git cat-file' command on the upstream of the
pipe.

* sk/test-commit-body-helper:
  t: use commit_body to extract commit message bodies
  test-lib-functions: add commit_body helper

diff --cc t/README
Simple merge

diff --cc t/t3404-rebase-interactive.sh
Simple merge

diff --cc t/t7500-commit-template-squash-signoff.sh
Simple merge

diff --cc t/t7600-merge.sh
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge