strbuf: use designated initializers in STRBUF_INIT

strbuf: use designated initializers in STRBUF_INIT

There are certain C99 features that might be nice to use in
our code base, but we've hesitated to do so in order to
avoid breaking compatibility with older compilers. But we
don't actually know if people are even using pre-C99
compilers these days.

One way to figure that out is to introduce a very small use
of a feature, and see if anybody complains. The strbuf code
is a good place to do this for a few reasons:

  - it always gets compiled, no matter which Makefile knobs
    have been tweaked.

  - it's very stable; this definition hasn't changed in a
    long time and is not likely to (so if we have to revert,
    it's unlikely to cause headaches)

If this patch can survive a few releases without complaint,
then we can feel more confident that designated initializers
are widely supported by our user base.  It also is an
indication that other C99 features may be supported, but not
a guarantee (e.g., gcc had designated initializers before
C99 existed).

And if we do get complaints, then we'll have gained some
data and we can easily revert this patch.

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