Refactor the call to curl in add-default-github-hooks

... in preparation for two-factor authentication...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
add-default-github-hooks.sh: support 2-factor auth

GitHub requires a special header when two-factor authentication is
enabled.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>