git: document GIT_NO_REPLACE_OBJECTS environment variable

git: document GIT_NO_REPLACE_OBJECTS environment variable

This variable is used as the primary way to disable the object
replacement mechanism, with the "--no-replace-objects" command line
option as an end-user visible way to set it, but has not been
documented.

The original reason why it was left undocumented might be because it
was meant as an internal implementation detail, but the thing is,
that our tests use the environment variable directly without the
command line option, and there certainly are folks who learned its
use from there, making it impossible to deprecate or change its
behaviour by now.

Add documentation and note that for this variable, unlike many
boolean-looking environment variables, only the presence matters,
not what value it is set to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>