Add absolute -> relative mode

[elliotgoodrich]

As mentioned here, we have an issue if Ninja uses absolute paths (CMake generates absolute paths for inputs for example) and those paths contain the name of the workspace, which differs across machines, e.g. C:/myserver1/projectX and C:/myserver73/projectX).

To fix this we could introduce a new mode --make-relative to trimja, which would:

1. Rewrite all inputs, outputs, order-only dependencies, and validations as relative if they were absolute. When Ninja runs against this new build file it will generate the build commands and build log using the relative path. There's a possibility that commands here fail because they expect an absolute path or have a different working directory.
2. Rewrite the .ninja_deps file and replace all absolute paths with relative ones. depfile generating commands can (and do) generate absolute paths so these need to be updated.