t: use commit_body to extract commit message bodies

t: use commit_body to extract commit message bodies

Replace the "git cat-file commit | sed" idiom with commit_body across the
test suite: 61 sites in 12 files, plus one local helper that wrapped the
same idiom. The idiom appears in four equivalent spellings -- piped or
written to a file first, "sed -e" or plain "sed", "\$" or "$" in the
address -- all producing byte-identical output; they all collapse to the
same commit_body call.

t7509-commit-authorship.sh defined its own local message_body() helper
around the idiom instead of spelling it out at each call site; remove the
helper and convert its six call sites to commit_body directly.

Two sites needed more than a mechanical substitution:

* t7600.sh ("merge --no-ff --edit") greps the raw commit object for a
  phrase before stripping its header for the final comparison. The
  phrase is part of the commit body, not the header, so the grep can
  run against the already-stripped body instead, letting both steps
  share one commit_body call.

* t3900-i18n-commit.sh pipes the stripped body into "iconv" to test
  re-encoding. Piping commit_body's output into "iconv" would reintroduce
  an exit-code hole one line after removing it elsewhere, so this site
  writes the body to a file first and reads that, keeping the &&-chain
  intact.

Some greps for sed -e "1,/^\*$/d" left unconverted, as they are not extracting a commit's message body:

* t9001-send-email.sh strips mail headers from a message file, not a
  commit object.
* t1450-fsck.sh strips the header off a hand-built commit object while
  constructing a malformed one for fsck to reject.
* t4014-format-patch.sh runs the same sed address on a ".patch" file,
  with an additional expression.

All converted files pass in full, and a deliberately failing
"git cat-file" now fails a converted test that previously passed.

Signed-off-by: Shlok Kulshreshtha <redacted>
Signed-off-by: Junio C Hamano <redacted>