pre-push.sample: Make the script executable

githooks(5) says that "[...]the .sample files are executable by default"
which was not true.

Signed-off-by: Wieland Hoffmann <themineo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/templates/hooks--pre-push.sample b/templates/hooks--pre-push.sample
old mode 100644
new mode 100755