tests: prepare for a world without whatchanged

tests: prepare for a world without whatchanged

Some tests on fast-import run "git whatchanged" without even
checking the output from the command.  It is tempting to remove the
calls altogether since they are not doing anything useful, but they
presumably were added there while the tests were developed to manually
sanity check which paths were touched.

Replace these calls with "git log --raw", which is a rough
equivalent in the more modern Git.

This does not remove "git whatchanged", but we no longer have to
worry about adjusting these places when we eventually do.

Helped-by: Elijah Newren <newren@gmail.com>
[en: log message]
Signed-off-by: Junio C Hamano <gitster@pobox.com>