submodule: avoid auto-discovery in prepare_submodule_repo_env()

The function is used to set up the environment variable used in a
subprocess we spawn in a submodule directory.  The callers set up a
child_process structure, find the working tree path of one submodule
and set .dir field to it, and then use start_command() API to spawn
the subprocess like "status", "fetch", etc.

When this happens, we expect that the ".git" (either a directory or
a gitfile that points at the real location) in the current working
directory of the subprocess MUST be the repository for the submodule.

If this ".git" thing is a corrupt repository, however, because
prepare_submodule_repo_env() unsets GIT_DIR and GIT_WORK_TREE, the
subprocess will see ".git", thinks it is not a repository, and
attempt to find one by going up, likely to end up in finding the
repository of the superproject.  In some codepaths, this will cause
a command run with the "--recurse-submodules" option to recurse
forever.

By exporting GIT_DIR=.git, disable the auto-discovery logic in the
subprocess, which would instead stop it and report an error.

The test illustrates existing problems in a few callsites of this
function.  Without this fix, "git fetch --recurse-submodules", "git
status" and "git diff" keep recursing forever.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

submodule.c

@@ -1160,4 +1160,5 @@ void prepare_submodule_repo_env(struct argv_array *out)
 		if (strcmp(*var, CONFIG_DATA_ENVIRONMENT))
 			argv_array_push(out, *var);
 	}
+	argv_array_push(out, "GIT_DIR=.git");
 }

t/t5526-fetch-submodules.sh

@@ -485,4 +485,39 @@ test_expect_success 'fetching submodules respects parallel settings' '
 	)
 '
 
+test_expect_success 'fetching submodule into a broken repository' '
+	# Prepare src and src/sub nested in it
+	git init src &&
+	(
+		cd src &&
+		git init sub &&
+		git -C sub commit --allow-empty -m "initial in sub" &&
+		git submodule add -- ./sub sub &&
+		git commit -m "initial in top"
+	) &&
+
+	# Clone the old-fashoned way
+	git clone src dst &&
+	git -C dst clone ../src/sub sub &&
+
+	# Make sure that old-fashoned layout is still supported
+	git -C dst status &&
+
+	# "diff" would find no change
+	git -C dst diff --exit-code &&
+
+	# Recursive-fetch works fine
+	git -C dst fetch --recurse-submodules &&
+
+	# Break the receiving submodule
+	rm -f dst/sub/.git/HEAD &&
+
+	# NOTE: without the fix the following tests will recurse forever!
+	# They should terminate with an error.
+
+	test_must_fail git -C dst status &&
+	test_must_fail git -C dst diff &&
+	test_must_fail git -C dst fetch --recurse-submodules
+'
+
 test_done