t8003: check output of coalesced blame

t8003: check output of coalesced blame

Commit f0cbe742f4 (blame: add a test to cover blame_coalesce(),
2019-06-20) added a test case where blame can usefully coalesce two
groups of lines. But since it relies on the normal blame output, it only
exercises the code and can't tell whether the lines were actually
joined into a single group.

However, by using --porcelain output, we can see how git-blame considers
the groupings (and likewise how the coalescing might have a real
user-visible impact for a tool that uses the porcelain-output
groupings). This lets us confirm that we are indeed coalescing correctly
(and the fact that this test case requires coalescing can be verified by
dropping the call to blame_coalesce(), causing the test to fail).

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