When reference-cache is enabled, shallow fetches (fetch-depth > 0) are
counterproductive because objects are served from the local cache.
Shallow negotiation only adds network latency without saving bandwidth.
If fetch-depth was not explicitly set by the user, it is automatically
overridden to 0. If explicitly set, a warning is emitted explaining
the performance impact.
Signed-off-by: Michael Wyraz <mw@brick4u.de>
