http-push: add back underscore separator before lock token

http-push: add back underscore separator before lock token

817d14a (http-push: refactor request url creation, 2009-01-31) removed the
underscore separator between the object path and the appended lock token.

This patch adds it back.

This would be keeping in line with the aforementioned patch's objective
of refactoring, without changing the behaviour and effect, of the code.

This would also be useful for testing if the lock token has been
indeed appended to the object url.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>