Source Link should emit a warning if files are ignored by the global gitignore file

[0xced]

I struggled to figure out that a non-global .gitignore file is required for Source Link to properly work with untracked files. A warning from Source Link would have been very useful.

[tmat]

Warning makes sense. We could also make global .gitignore working but I'd argue that's not a good practice in general since it makes the build non-deterministic (i.e. changing behavior depending on what machine the build is run on).