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>