This reworks PerforceClient to make use of the new diff tools and

UnifiedDiffWriter to generate diffs.
Now, the entire diff operation uses a single UnifiedDiffWriter to

generate the result. This is more efficient than building and joining a

bunch of lists of byte strings. It also helps create more standardized

output, taking care of issues like \r\r\n newlines in the process.
Diffs of files are performed using the new diff tool support, which

simplifies logic and should provide wider compatibility as new diff tool

backends are implemented.
A few fixes are also here for unsafe variable assumptions and dead code.

Unit tests pass.