FWIW, I see some typos. Otherwise this patch along with your "git:
document GIT_NO_REPLACE_OBJECTS environment variable" one both LGTM for
wording/readability. I must defer to Peff and others for "is this patch
actually doing the right thing?" ;).

Junio C Hamano [off-list ref] writes:

Modeling after how the `--no-replace-objects` option is made usable
across subprocess spawning (e.g., cURL based remote helpers are
spawned as a separate process while running "git fetch"), allow the
`--no-lazy-fetch` option to be passed across process boundary.

s/boundary/boundaries

Do not model how the value of GIT_NO_REPLACE_OBJECTS environment
variable is ignored, though.  Just use the usual git_env_bool() to
allow "export GIT_NO_LAZY_FETCH=0" and "unset GIT_NO_LAZY_FETCH"
to be equivalents.

s/equivalents/equivalent