Add text view for diff

Rather than define a complex JSON format for representing diffs (or,
more likely, copying Gerrit Code Review), use the standard textual
diff format, base64-encoded to avoid decoding by browsers. Tools
already exist to parse this output.

Bug: 55
Change-Id: I985ca6e70488ce1ea426b8c9773e57c11567d8a6
2 files changed