Fix diff view comparing a deleted page with itself (#4635)

When opening the diff of a deleted page without explicit rev parameters
(?do=diff), Diff::handle() resolved the older side via getRevisions

Fix diff view comparing a deleted page with itself (#4635)

When opening the diff of a deleted page without explicit rev parameters
(?do=diff), Diff::handle() resolved the older side via getRevisions(0, 1).
That helper only skips the current revision when the item file still
exists, so for a deleted page it returned the deletion entry itself and
the view ended up comparing the current revision with itself ("no way to
compare when less than two revisions", empty diff).

Use getRelativeRevision($rev2, -1) instead, which returns the revision
immediately before the current one regardless of whether the page file
is still present. This covers both pages deleted through DokuWiki and
externally deleted pages (whose synthesized deletion entry is now
persisted to the changelog).

Add PageDiffTest and PageChangeLogTest covering the resolved revision
pair and the underlying changelog walk-back for both deletion kinds.

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