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

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