test: use a more verbose output in the test
While debugging some test failure, I released the test never checks if the
relevant changesets were preserved. So I am updating the test from `hg parents`
usage to `hg log -G` with a special template. This increase the area covered by
the test and clarify the test failures.
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed