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>

argv-array.c

@@ -0,0 +1,51 @@
+#include "cache.h"
+#include "argv-array.h"
+#include "strbuf.h"
+
+static const char *empty_argv_storage = NULL;
+const char **empty_argv = &empty_argv_storage;
+
+void argv_array_init(struct argv_array *array)
+{
+	array->argv = empty_argv;
+	array->argc = 0;
+	array->alloc = 0;
+}
+
+static void argv_array_push_nodup(struct argv_array *array, const char *value)
+{
+	if (array->argv == empty_argv)
+		array->argv = NULL;
+
+	ALLOC_GROW(array->argv, array->argc + 2, array->alloc);
+	array->argv[array->argc++] = value;
+	array->argv[array->argc] = NULL;
+}
+
+void argv_array_push(struct argv_array *array, const char *value)
+{
+	argv_array_push_nodup(array, xstrdup(value));
+}
+
+void argv_array_pushf(struct argv_array *array, const char *fmt, ...)
+{
+	va_list ap;
+	struct strbuf v = STRBUF_INIT;
+
+	va_start(ap, fmt);
+	strbuf_vaddf(&v, fmt, ap);
+	va_end(ap);
+
+	argv_array_push_nodup(array, strbuf_detach(&v, NULL));
+}
+
+void argv_array_clear(struct argv_array *array)
+{
+	if (array->argv != empty_argv) {
+		int i;
+		for (i = 0; i < array->argc; i++)
+			free((char **)array->argv[i]);
+		free(array->argv);
+	}
+	argv_array_init(array);
+}