Finding myself in this exact scenario, I've take it upon myself to write such a custom importer: export-surround-to-git
This method is capable of preserving complete history, timestamps, authors, comments, branches, snapshots, etc.
This script has been tested in a limited number of scenarios, and has proven successful so far. While still a work-in-progress, this may be what you're looking for. It's open-source--so feel free to make improvements to the script on your own!
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…