[MERGE] Remove supports_single_merge_revision cruft from log

[Ian Clatworthy]

This was discussed on the list 2 months back and it was agreed
by all concerned (Kent Gibson, Martin Pool, Vincent Ladeuil) that
it was only used inside bzrlib and no longer had any relevance.

The most important bit here is the removal of a XFAIL test
that I see every time I run "bzr selftest log", many times
every week. It also simplifies the log code. As a side effect
of this change, another test also needed to be removed. It
was testing an assertion that isn't true and just happened to
"pass" because the obsolete code being removed was triggering
a BzrCommandError. Sigh.

In the reviewer has any concerns, I can dig up the email
discussion (but I can't be bothered right now).

Ian C.
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