filelog: don't crash on invalid copy metadata (issue5748) "copy" and "copyrev" are both supposed to appear next to each other. However, a user report demonstrated a crash that indicates that something in the wild is producing "copy" without "copyrev" (probably `hg convert`). While we should definitely fix the source of the bad metadata, the bad code causing the crash is already in the wild and who knows how many repositories are impacted. So let's be more defensive when accessing the file revision metadata.

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