transport-helper: avoid reading past end-of-string

We detect the "import-marks" capability by looking for that
string, but _without_ a trailing space. Then we skip past it
using strlen("import-marks "), with a space. So if a remote
helper gives us exactly "import-marks", we will read past
the end-of-string by one character.

This is unlikely to be a problem in practice, because such
input is malformed in the first place, and because there is
a good chance that the string has an extra NUL terminator
one character after the original (because it formerly had a
newline in it that we parsed off).

We can fix it by using skip_prefix with "import-marks ",
with the space. The other form appears to be a typo from
a515ebe (transport-helper: implement marks location as
capability, 2011-07-16); "import-marks" has never existed
without an argument, and it should match the "export-marks"
definition above.

Speaking of which, we can also use skip_prefix in a few
other places while we are in the function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

transport-helper.c

@@ -153,7 +153,7 @@ static struct child_process *get_helper(struct transport *transport)
 	write_constant(helper->in, "capabilities\n");
 
 	while (1) {
-		const char *capname;
+		const char *capname, *arg;
 		int mandatory = 0;
 		if (recvline(data, &buf))
 			exit(128);
@@ -183,19 +183,19 @@ static struct child_process *get_helper(struct transport *transport)
 			data->export = 1;
 		else if (!strcmp(capname, "check-connectivity"))
 			data->check_connectivity = 1;
-		else if (!data->refspecs && starts_with(capname, "refspec ")) {
+		else if (!data->refspecs && skip_prefix(capname, "refspec ", &arg)) {
 			ALLOC_GROW(refspecs,
 				   refspec_nr + 1,
 				   refspec_alloc);
-			refspecs[refspec_nr++] = xstrdup(capname + strlen("refspec "));
+			refspecs[refspec_nr++] = xstrdup(arg);
 		} else if (!strcmp(capname, "connect")) {
 			data->connect = 1;
 		} else if (!strcmp(capname, "signed-tags")) {
 			data->signed_tags = 1;
-		} else if (starts_with(capname, "export-marks ")) {
-			data->export_marks = xstrdup(capname + strlen("export-marks "));
-		} else if (starts_with(capname, "import-marks")) {
-			data->import_marks = xstrdup(capname + strlen("import-marks "));
+		} else if (skip_prefix(capname, "export-marks ", &arg)) {
+			data->export_marks = xstrdup(arg);
+		} else if (skip_prefix(capname, "import-marks ", &arg)) {
+			data->import_marks = xstrdup(arg);
 		} else if (starts_with(capname, "no-private-update")) {
 			data->no_private_update = 1;
 		} else if (mandatory) {