revlog: return lazy set from findcommonmissing
When computing the commonmissing, it greedily computes the entire set
immediately. On a large repo where the majority of history is irrelevant, this
causes a significant slow down.
Replacing it with a lazy set makes amend go from 11 seconds to 8.7 seconds.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.