Merge branch 'ps/cache-tree-w-broken-index-entry'

Fail gracefully instead of crashing when attempting to write the contents of a corrupt in-core index as a tree object. * ps/cache-tree-w-broken-index-entry: unpack-trees: detect mismatching number of cache-tree/index entries cache-tree: detect mismatching number of index entries cache-tree: refactor verification to return error codes