t8003: factor setup out of coalesce test

t8003: factor setup out of coalesce test

In preparation for adding more tests of blame's coalesce code, let's
split the setup out from the first test, and give each of the commits
a more meaningful name:

  - $orig for the original source that added the lines

  - $split for the version where they are split apart

  - $final for the final version that re-joins them

That's not strictly necessary, but makes the follow-on tests less
brittle than relying on HEAD^, etc, to name the commits.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>