submodule init: fail gracefully with a missing .gitmodules file

When there is no .gitmodules file availabe to initialize a submodule
from, `submodule_from_path` just returns NULL. We need to check for
that and abort gracefully. When `submodule init` was implemented in shell,
a missing .gitmodules file would result in an error message

    No url found for submodule path '%s' in .gitmodules

Replicate that error message for now.

When the .gitmodules file is missing we can probably fail even earlier
for all of the submodules with an improved error message.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -314,13 +314,17 @@ static void init_submodule(const char *path, const char *prefix, int quiet)
 	/* Only loads from .gitmodules, no overlay with .git/config */
 	gitmodules_config();
 
-	sub = submodule_from_path(null_sha1, path);
-
 	if (prefix) {
 		strbuf_addf(&sb, "%s%s", prefix, path);
 		displaypath = strbuf_detach(&sb, NULL);
 	} else
-		displaypath = xstrdup(sub->path);
+		displaypath = xstrdup(path);
+
+	sub = submodule_from_path(null_sha1, path);
+
+	if (!sub)
+		die(_("No url found for submodule path '%s' in .gitmodules"),
+			displaypath);
 
 	/*
 	 * Copy url setting when it is not set yet.

t/t7400-submodule-basic.sh

@@ -18,6 +18,14 @@ test_expect_success 'setup - initial commit' '
 	git branch initial
 '
 
+test_expect_success 'submodule init aborts on missing .gitmodules file' '
+	test_when_finished "git update-index --remove sub" &&
+	git update-index --add --cacheinfo 160000,$(git rev-parse HEAD),sub &&
+	# missing the .gitmodules file here
+	test_must_fail git submodule init 2>actual &&
+	test_i18ngrep "No url found for submodule path" actual
+'
+
 test_expect_success 'configuration parsing' '
 	test_when_finished "rm -f .gitmodules" &&
 	cat >.gitmodules <<-\EOF &&