[clang-doc] If `clang-doc` is invoked with `--repository` from a different directory than the target, the remote URLs aren't serialized

Example invocation:

```
~/llvm-project/cmake-build-release/bin/clang-doc --format=html --executor=all-TUs --output=~/doxygen/clang-doc-output --repository=https://github.com/doxygen/doxygen/blob/master/ ~/doxygen/build/compile_commands.json
```

Running that from a different directory (say your pwd is llvm, and you want documentation for another project) does not result in the expected URL information being serialized in JSON. This seems like unexpected behavior regarding invocation, say like running clang-doc on a server.