book: Always overwrite history to reduce repo size.

.github/workflows/book.yml

@@ -64,3 +64,4 @@ jobs:
               with:
                 github_token: ${{ secrets.GITHUB_TOKEN }}
                 branch: gh-pages
+                force: true

doc/Makefile

@@ -30,6 +30,6 @@ clean:
 	rm -rf book
 
 github: all
-	ghp-import -m "Generate mdbook" -b gh-pages book/
+	./ghp_push.py
 
 .PHONY: $(JSONRPC) all clean github

doc/ghp_push.py

@@ -0,0 +1,3 @@
+#!/usr/bin/env python3
+from ghp_import import ghp_import
+ghp_import("book", branch="gh-pages", no_history=True, mesg="Generate mdbook")