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.
% file replaced with directory
adding a
% should fail - would corrupt dirstate
abort: file 'a' in dirstate clashes with 'a/a'
% directory replaced with file
adding a/a
% should fail - would corrupt dirstate
abort: directory 'a' already in dirstate
% directory replaced with file
adding b/c/d
% should fail - would corrupt dirstate
abort: directory 'b' already in dirstate