[gtk-osx] Silence git warning about how to merge a pull.

[John Ralls <jralls src gnome org>]

commit 9223c4a102e188947b27dc68359c29d9ea5999c7
Author: John Ralls <jralls ceridwen us>
Date:   Fri Aug 27 10:18:07 2021 -0700

    Silence git warning about how to merge a pull.

 gtk-osx-setup.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk-osx-setup.sh b/gtk-osx-setup.sh
index fdac7f7c..344a5f35 100755
--- a/gtk-osx-setup.sh
+++ b/gtk-osx-setup.sh
@@ -83,7 +83,7 @@ if test ! -x "$PYENV_INSTALL_ROOT/libexec/pyenv"; then
   git clone $GITHUB/pyenv/pyenv.git "$PYENV_INSTALL_ROOT"
 else
     pushd "$PYENV_INSTALL_ROOT"
-    git pull
+    git pull --ff-only
     popd
 fi