make git-bugreport a builtin

make git-bugreport a builtin

There's no reason that bugreport has to be a separate binary. And since
it links against libgit.a, it has a rather large disk footprint. Let's
make it a builtin, which reduces the size of a stripped installation
from 24MB to 22MB.

This also simplifies our Makefile a bit. And we can take advantage of
builtin niceties like RUN_SETUP_GENTLY.

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