rev-branch-cache: properly ignores unaligned trailing data Previously, trailing data could lead to crash and would be written back to disk, disaligning all new data… This is no longer the cases. This was detected while playing with branchmap-v3 that access the rev-branch-cache much more aggressively.

rev-branch-cache: stop pretending we will overwrite data when we don't We were issuing a message about overwriting data even when we were about to write 0 bytes in pratice. This is silly. Instead we point at the extra data remaining in the file (in case someone is using debug to debug something).

rev-branch-cache: fix message about overwritten data If there is data at the end we ignore, we should not count them as overwritten. This reveal that we something don't overwrite anything, this will be taken car of in the next changeset.