git_remote_helpers: force rebuild if python version changes

When different version of python are used to build via distutils, the
behaviour can change.  Detect changes in version and pass --force in
this case.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git_remote_helpers/.gitignore

@@ -1,2 +1,3 @@
+/GIT-PYTHON-VERSION
 /build
 /dist

git_remote_helpers/Makefile

@@ -25,8 +25,14 @@ PYLIBDIR=$(shell $(PYTHON_PATH) -c \
 	 "import sys; \
 	 print('lib/python%i.%i/site-packages' % sys.version_info[:2])")
 
+py_version=$(shell $(PYTHON_PATH) -c \
+	'import sys; print("%i.%i" % sys.version_info[:2])')
+
 all: $(pysetupfile)
-	$(QUIET)$(PYTHON_PATH) $(pysetupfile) $(QUIETSETUP) build
+	$(QUIET)test "$$(cat GIT-PYTHON-VERSION 2>/dev/null)" = "$(py_version)" || \
+	flags=--force; \
+	$(PYTHON_PATH) $(pysetupfile) $(QUIETSETUP) build $$flags
+	$(QUIET)echo "$(py_version)" >GIT-PYTHON-VERSION
 
 install: $(pysetupfile)
 	$(PYTHON_PATH) $(pysetupfile) install --prefix $(DESTDIR_SQ)$(prefix)