tests: remove some direct access to .git/logs

tests: remove some direct access to .git/logs

Alternate refs backends might store reflogs somewhere other than
.git/logs.  Change most test code that directly accesses .git/logs to
instead use git reflog commands.

There are still a few tests which need direct access to reflogs: to
check reflog permissions, to manually create reflogs from scratch, to
save/restore reflogs, to check the format of raw reflog data, and to
remove not just reflog contents, but the reflogs themselves. All cases
which don't need direct access have been modified.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>