localrepo: add optional validation (defaults to off) for incoming changes
This verifies that all manifests are present for incoming changes,
and all files for those manifests are also present. This is a simple
first-pass, and could be better, but seems like a valuable thing to
have, as I've seen pushes in the past that propagated revlog corruption.
foo renamed from bar:26d3ca0dfd18e44d796b564e38dd173c9668d3a9
rev offset length base linkrev nodeid p1 p2
0 0 6 0 0 26d3ca0dfd18 000000000000 000000000000
1 6 7 1 1 d267bddd54f7 26d3ca0dfd18 000000000000