sagemathgh-39392: Test that the Sage git repo is clean.

    
Test that the Sage git repo is clean

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Add a doctest which will check whether the Sage git repo is clean. The
test is based on the top answer at
https://unix.stackexchange.com/questions/155046/determine-if-git-
working-directory-is-clean-from-a-script.

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: sagemath#39392
Reported by: John H. Palmieri
Reviewer(s):

Makefile

@@ -253,9 +253,19 @@ TEST_TARGET = $@
 
 TEST = ./sage -t --logfile=$(TEST_LOG) $(TEST_FLAGS) --optional=$(TEST_OPTIONAL) $(TEST_FILES)
 
+test-git-no-uncommitted-changes:
+	@UNCOMMITTED=$$(git status --porcelain); \
+	if [ -n "$$UNCOMMITTED" ]; then \
+	    echo "Error: the git repo has uncommitted changes:"; \
+	    echo "$$UNCOMMITTED"; \
+	    echo; \
+	    exit 1; \
+	fi
+
 test: all
 	@echo '### make $(TEST_TARGET): Running $(TEST)' >> $(TEST_LOG)
-	$(TEST)
+	$(TEST); \
+	$(MAKE) test-git-no-uncommitted-changes
 
 check:
 	@$(MAKE) test
@@ -302,7 +312,8 @@ ptestoptionallong:
 test-nodoc: TEST_OPTIONAL := $(TEST_OPTIONAL),!sagemath_doc_html,!sagemath_doc_pdf
 test-nodoc: build
 	@echo '### make $(TEST_TARGET): Running $(TEST)' >> $(TEST_LOG)
-	$(TEST)
+	$(TEST); \
+	$(MAKE) test-git-no-uncommitted-changes
 
 check-nodoc:
 	@$(MAKE) test-nodoc
@@ -387,5 +398,6 @@ list:
 	misc-clean bdist-clean distclean bootstrap-clean maintainer-clean \
 	test check testoptional testall testlong testoptionallong testallong \
 	ptest ptestoptional ptestall ptestlong ptestoptionallong ptestallong \
+	test-git-no-uncommitted-changes \
 	list \
 	doc-clean clean sagelib-clean build-clean