string-list: add string_list initializer helper function

The string-list API has STRING_LIST_INIT_* macros to be used
to define variables with initializers, but lacks functions
to initialize an uninitialized piece of memory to be used as
a string-list at the run-time.
Introduce `string_list_init()` function for that.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-string-list.txt

@@ -68,6 +68,11 @@ Functions
 
 * General ones (works with sorted and unsorted lists as well)
 
+`string_list_init`::
+
+	Initialize the members of the string_list, set `strdup_strings`
+	member according to the value of the second parameter.
+
 `filter_string_list`::
 
 	Apply a function to each item in a list, retaining only the