fix(authors): avoid fetching blobs for shallow history

[njzjz-bot]

Problem

• The authors directive fetches full history in shallow Read the Docs checkouts.
• A plain git fetch --unshallow can download repository blob contents even though git shortlog only needs commit metadata.

Change

• Split shallow-repository detection and history fetching into helper functions.
• Fetch unshallowed history with --filter=blob:none so partial-clone-capable servers can avoid downloading blobs.
• Warn instead of failing hard if the filtered fetch is unavailable.

Verification

• python3 -m py_compile deepmodeling_sphinx/authors.py
• git diff --check
• Manual shallow clone smoke test invoking get_authors() with READTHEDOCS=True

Authored by OpenClaw (model: custom-chat-jinzhezeng-group/gpt-5.5)

[njzjz]

lgtm https://deepmodeling--77.org.readthedocs.build/projects/deepmodeling-sphinx/en/77/credits.html