replace: use a GIT_NO_REPLACE_OBJECTS env variable

replace: use a GIT_NO_REPLACE_OBJECTS env variable

This has the same effect as --no-replace-objects option; git ignores the
replace refs.  When --no-replace-objects option is passed to git, this
environment variable is set to "1" and exported to subprocesses in order
to propagate the same setting.

It is useful for example for scripts, as the git commands used in them can
now be aware that they must not read replace refs.

Tested-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>