Roll open-vcdiff 21d7d0b9:2b9bd1fe

Picks up cmake support and a new public interface, as well as some changes to
the standalone binary targets.
Full log: https://chromium.googlesource.com/external/github.com/google/open-vcdiff/+log/21d7d0b9..2b9bd1fe

NOPRESUBMIT because updated generated config files have weird copyright headers in them.

NOPRESUBMIT=true
BUG=

Review-Url: https://codereview.chromium.org/2204683003
Cr-Commit-Position: refs/heads/master@{#411031}
8 files changed