build-python: Discard local changes before checking out a new revision

Even if we don't patch cpython-mingw any longer, we still run
autoreconf on it (and libffi), which can cause changes to
committed files. If trying to check out a different version of
the source, this would fail unless the local changes have been
discarded.

build-python.sh

@@ -64,6 +64,7 @@ fi
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT_LIBFFI" ]; then
     cd libffi
     [ -z "$SYNC" ] || git fetch
+    git reset --hard
     git checkout $LIBFFI_VERSION
     autoreconf -vfi
     cd ..
@@ -117,6 +118,7 @@ fi
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT_PYTHON" ]; then
     cd cpython-mingw
     [ -z "$SYNC" ] || git fetch
+    git reset --hard
     git checkout $PYTHON_VERSION_MINGW
     autoreconf -vfi
     cd ..