Add tests to check the behavior of fetching from a repository which
changes between rounds of negotiation (for example, when different
servers in a load-balancing agreement participate in the same stateless
RPC negotiation). This forms a baseline of comparison to the ref-in-want
functionality (which will be introduced to the client in subsequent
commits), and ensures that subsequent commits do not change existing
behavior.

As part of this effort, a mechanism to substitute strings in a single
HTTP response is added.
Signed-off-by: NBrandon Williams <bmwill@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>