refactor argv_array into generic code

The submodule code recently grew generic code to build a
dynamic argv array. Many other parts of the code can reuse
this, too, so let's make it generically available.

There are two enhancements not found in the original code:

  1. We now handle the NULL-termination invariant properly,
     even when no strings have been pushed (before, you
     could have an empty, NULL argv). This was not a problem
     for the submodule code, which always pushed at least
     one argument, but was not sufficiently safe for
     generic code.

  2. There is a formatted variant of the "push" function.
     This is a convenience function which was not needed by
     the submodule code, but will make it easier to port
     other users to the new code.

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

Documentation/technical/api-argv-array.txt

@@ -0,0 +1,46 @@
+argv-array API
+==============
+
+The argv-array API allows one to dynamically build and store
+NULL-terminated lists.  An argv-array maintains the invariant that the
+`argv` member always points to a non-NULL array, and that the array is
+always NULL-terminated at the element pointed to by `argv[argc]`. This
+makes the result suitable for passing to functions expecting to receive
+argv from main(), or the link:api-run-command.html[run-command API].
+
+The link:api-string-list.html[string-list API] is similar, but cannot be
+used for these purposes; instead of storing a straight string pointer,
+it contains an item structure with a `util` field that is not compatible
+with the traditional argv interface.
+
+Each `argv_array` manages its own memory. Any strings pushed into the
+array are duplicated, and all memory is freed by argv_array_clear().
+
+Data Structures
+---------------
+
+`struct argv_array`::
+
+	A single array. This should be initialized by assignment from
+	`ARGV_ARRAY_INIT`, or by calling `argv_array_init`. The `argv`
+	member contains the actual array; the `argc` member contains the
+	number of elements in the array, not including the terminating
+	NULL.
+
+Functions
+---------
+
+`argv_array_init`::
+	Initialize an array. This is no different than assigning from
+	`ARGV_ARRAY_INIT`.
+
+`argv_array_push`::
+	Push a copy of a string onto the end of the array.
+
+`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`.
+
+`argv_array_clear`::
+	Free all memory associated with the array and return it to the
+	initial, empty state.