doc/Makefile: restore CI check for schema changes.

We now know the base reference, and we've rebased, so we can do a
simple diff.  Also, this means we can use a magic commit message
`No-schema-diff-check` to suppress false positives.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/Makefile

@@ -227,22 +227,22 @@ doc/index.rst: $(MANPAGES:=.md)
 	  python3 devtools/blockreplace.py doc/index.rst manpages --language=rst --indent "   " \
 	)
 
-# For CI to (very roughly!) check that we only deprecated fields, or labelled added ones
-# When running on GitHub (CI=true), we need to fetch origin/master
+# Overridden by GH CI if necessary.
+BASE_REF=master
 schema-added-check:
-	@if ! test -z $$CI; then git fetch origin master; fi; \
-	if git diff origin/master -- doc/schemas | grep -q '^+.*{' && ! git diff origin/master -- doc/schemas | grep -q '^+.*"added"'; then \
-		git diff origin/master -- doc/schemas; \
+	if git show --format=%B -s $(BASE_REF).. | grep -q '^No-schema-diff-check'; then echo $@ suppressed; exit 0; fi; \
+	if git diff $(BASE_REF) -- doc/schemas | grep -q '^+.*{' && ! git diff $(BASE_REF) -- doc/schemas | grep -q '^+.*"added"'; then \
+		git diff $(BASE_REF) -- doc/schemas; \
 		echo 'New schema fields must have "added": "vNEXTVERSION"' >&2; exit 1; \
 	fi
+
 schema-removed-check:
-	@if ! test -z $$CI; then git fetch origin master; fi; \
-	if git diff origin/master -- doc/schemas | grep -q '^-.*{' && ! git diff origin/master -- doc/schemas | grep -q '^-.*"deprecated"' && ! git diff origin/master -- doc/schemas | grep -q '^-.*EXPERIMENTAL_FEATURES'; then \
-		git diff origin/master -- doc/schemas ; \
+	if git show --format=%B -s $(BASE_REF).. | grep -q '^No-schema-diff-check'; then echo $@ suppressed; exit 0; fi; \
+	if git diff $(BASE_REF) -- doc/schemas | grep -q '^-.*{' && ! git diff $(BASE_REF) -- doc/schemas | grep -q '^-.*"deprecated"' && ! git diff $(BASE_REF) -- doc/schemas | grep -q '^-.*EXPERIMENTAL_FEATURES'; then \
+		git diff $(BASE_REF) -- doc/schemas ; \
 		echo 'Schema fields must be "deprecated", with version, not removed' >&2; exit 1; \
 	fi
 
 schema-diff-check: schema-added-check schema-removed-check
 
-# This breaks current commit; will revert after.
-#check-source: schema-diff-check
+check-source: schema-diff-check