index-pack: clarify the breached limit

index-pack: clarify the breached limit

As a small courtesy to users, report what limit was breached. This
is especially useful when a push exceeds a server-defined limit, since
the user is unlikely to have configured the limit (their host did).
Also demonstrate the human-readable message in a test.

Helped-by: Taylor Blau <me@ttaylorr.com>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Matt Cooper <vtbassmatt@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>