Fix relative paths in Git revision headers

[fallintoplace]

Description

What

• Fix relative paths in rapids_cmake_write_git_revision_file.

Why

• Nested paths fail CMake generation. Plain filenames write headers into the source directory.

Implementation

• Use the resolved build path for output, byproducts, and includes.
• Add regression coverage for plain filenames and nested paths with spaces.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[copy-pr-bot]

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[KyleFromNVIDIA]

Closing as very few of our projects use this feature, and none hit the code path that's being fixed. If this issue comes up in RAPIDS, we can revisit.