git: extend --no-lazy-fetch to work across subprocesses

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 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.

Also do not model how the request is not propagated to subprocesses
we spawn (e.g. "git clone --local" that spawns a new process to work
in the origin repository, while the original one working in the
newly created one) by the "--no-replace-objects" option, as this "do
not lazily fetch from the promisor" is more about a per-request
debugging aid, not "this repository's promisor should not be relied
upon" property specific to a repository.

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

t/t0410-partial-clone.sh

@@ -665,6 +665,21 @@ test_expect_success 'lazy-fetch when accessing object not in the_repository' '
 	git -C partial.git rev-list --objects --missing=print HEAD >out &&
 	grep "[?]$FILE_HASH" out &&
 
+	# The no-lazy-fetch mechanism prevents Git from fetching
+	test_must_fail env GIT_NO_LAZY_FETCH=1 \
+		git -C partial.git cat-file -e "$FILE_HASH" &&
+
+	# The same with command line option to "git"
+	test_must_fail git --no-lazy-fetch -C partial.git cat-file -e "$FILE_HASH" &&
+
+	# The same, forcing a subprocess via an alias
+	test_must_fail git --no-lazy-fetch -C partial.git \
+		-c alias.foo="!git cat-file" foo -e "$FILE_HASH" &&
+
+	# Sanity check that the file is still missing
+	git -C partial.git rev-list --objects --missing=print HEAD >out &&
+	grep "[?]$FILE_HASH" out &&
+
 	git -C full cat-file -s "$FILE_HASH" >expect &&
 	test-tool partial-clone object-info partial.git "$FILE_HASH" >actual &&
 	test_cmp expect actual &&