meson: add rule to run 'git clang-format'

meson: add rule to run 'git clang-format'

The Makefile has a 'style' rule to run 'git clang-format'. While Meson
intrinsically supports a 'clang-format' target, which can be run when
using the ninja backend by running 'ninja clang-format', this runs the
formatting on all existing files.

Our Meson build doesn't yet support a way to run 'git clang-format',
which runs the formatter between the working directory and commit
provided. Add a new 'style' target to Meson to mimic the target in the
Makefile.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>