Ensure the source repo in the cache is in a clean state before building

Summary:
This ensures the script can easily recover from common failures, such
as an incomplete cherry-pick, which leaves the git repo in a bad state and prevents
further pulling of latest changes.

Test Plan:
Edit some files in the cache.

fetch_documentation.sh

The script should complete rather than fail.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7784