sphinx_git extension does not declare if it is safe for parallel reading

[th0ger]

When running sphinx in parallel mode sphinx-build -j auto I get:

WARNING: the sphinx_git extension does not declare if it is safe for parallel reading, assuming it isn't - please ask the extension author to check and make it explicit
WARNING: doing serial read

As this is not only a warning, but also seems to throw parallel builds into serial mode, please fix this.

[th0ger]

Please consider adding parallel_read_safe.

[arwedus]

@OddBloke : Did you abandon this project? If now, please review the patch waiting to be merged. We want to pursue a zero-warning strategy for Sphinx in our project, which is why we want to avoid this warning.