Have the streaming interface and other codepaths more carefully
examine for corrupt objects.

* jk/check-corrupt-objects-carefully:
  clone: leave repo in place after checkout errors
  clone: run check_everything_connected
  clone: die on errors from unpack_trees
  add tests for cloning corrupted repositories
  streaming_write_entry: propagate streaming errors
  add test for streaming corrupt blobs
  avoid infinite loop in read_istream_loose
  read_istream_filtered: propagate read error from upstream
  check_sha1_signature: check return value from read_istream
  stream_blob_to_fd: detect errors reading from stream