This introduces the first diff tool, GNUDiffTool. This wraps around

GNU Diff, properly checking search paths, building the correct command

line, and interpreting results.
If there are multiple diff executables found in the path, this will

end up checking each until it finds GNU Diff or exhausts all options.
The implementation handles all documented GNU Diff exit codes, providing

text vs. binary flags in the result (handling old legacy binary file

indicators) and all diff output.
This will be a step up from a lot of the GNU diff executions we

currently have in the codebase, and will let us remove a lot of

different forms of error handling, binary file support, and execute name

determination.

Unit tests passed.
Tested this with some in-progress changes to switch SCMClients over. They

successfully ran GNU Diff and handled the results.