Code tightening against a "wrong" object appearing where an object
of a different type is expected, instead of blindly assuming that
the connection between objects are correctly made.
* tb/unexpected:
rev-list: detect broken root trees
rev-list: let traversal die when --missing is not in use
get_commit_tree(): return NULL for broken tree
list-objects.c: handle unexpected non-tree entries
list-objects.c: handle unexpected non-blob entries
t: introduce tests for unexpected object types
t: move 'hex2oct' into test-lib-functions.sh