test-lib-functions: add commit_body helper

test-lib-functions: add commit_body helper

Extracting the message body of a commit -- running "git cat-file commit"
and stripping everything up to and including the first blank line with
"sed" -- is spelled out in about 60 places across the test suite.

Add a helper for it, so that the operation is written once instead of
being copied around.

The commit object goes to a temporary file rather than into a pipe,
because a pipeline reports only its last command's exit status, so a
failure of "git cat-file" would go unnoticed.

Signed-off-by: Shlok Kulshreshtha <diy2903@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>