submodule: ignore path and url attributes if they look like options

These can be used to inject options in an implementation which performs a
recursive clone by executing an external command via crafted url and path
attributes such that it triggers a local executable to be run.

The library is not vulnerable as we do not rely on external executables but a
user of the library might be relying on that so we add this protection.

This matches this aspect of git's fix for CVE-2018-17456.

src/submodule.c

@@ -1865,6 +1865,14 @@ static int get_value(const char **out, git_config *cfg, git_buf *buf, const char
 	return error;
 }
 
+static bool looks_like_command_line_option(const char *s)
+{
+	if (s && s[0] == '-')
+		return true;
+
+	return false;
+}
+
 static int submodule_read_config(git_submodule *sm, git_config *cfg)
 {
 	git_buf key = GIT_BUF_INIT;
@@ -1878,6 +1886,8 @@ static int submodule_read_config(git_submodule *sm, git_config *cfg)
 
 	if ((error = get_value(&value, cfg, &key, sm->name, "path")) == 0) {
 		in_config = 1;
+		/* We would warn here if we had that API */
+		if (!looks_like_command_line_option(value)) {
 	/*
 	 * TODO: if case insensitive filesystem, then the following strcmp
 	 * should be strcasecmp
@@ -1888,14 +1898,19 @@ static int submodule_read_config(git_submodule *sm, git_config *cfg)
 				sm->path = git__strdup(value);
 				GITERR_CHECK_ALLOC(sm->path);
 			}
+
+		}
 	} else if (error != GIT_ENOTFOUND) {
 		goto cleanup;
 	}
 
 	if ((error = get_value(&value, cfg, &key, sm->name, "url")) == 0) {
+		/* We would warn here if we had that API */
+		if (!looks_like_command_line_option(value)) {
 			in_config = 1;
 			sm->url = git__strdup(value);
 			GITERR_CHECK_ALLOC(sm->url);
+		}
 	} else if (error != GIT_ENOTFOUND) {
 		goto cleanup;
 	}