argv-array: add pop function

Sometimes we build a set of similar command lines, differing
only in the final arguments (e.g., "fetch --multiple"). To
use argv_array for this, you have to either push the same
set of elements repeatedly, or break the abstraction by
manually manipulating the array's internal members.

Instead, let's provide a sanctioned "pop" function to remove
elements from the end.

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

argv-array.c

@@ -49,6 +49,15 @@ void argv_array_pushl(struct argv_array *array, ...)
 	va_end(ap);
 }
 
+void argv_array_pop(struct argv_array *array)
+{
+	if (!array->argc)
+		return;
+	free((char *)array->argv[array->argc - 1]);
+	array->argv[array->argc - 1] = NULL;
+	array->argc--;
+}
+
 void argv_array_clear(struct argv_array *array)
 {
 	if (array->argv != empty_argv) {