argv-array: add a new "pushl" method

It can be convenient to push many strings in a single line
(e.g., if you are initializing an array with defaults). This
patch provides a convenience wrapper to allow this.

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

Documentation/technical/api-argv-array.txt

@@ -37,6 +37,11 @@ Functions
 `argv_array_push`::
 	Push a copy of a string onto the end of the array.
 
+`argv_array_pushl`::
+	Push a list of strings onto the end of the array. The arguments
+	should be a list of `const char *` strings, terminated by a NULL
+	argument.
+
 `argv_array_pushf`::
 	Format a string and push it onto the end of the array. This is a
 	convenience wrapper combining `strbuf_addf` and `argv_array_push`.