bugreport: gather git version and build info

bugreport: gather git version and build info

Knowing which version of Git a user has and how it was built allows us
to more precisely pin down the circumstances when a certain issue
occurs, so teach bugreport how to tell us the same output as 'git
version --build-options'.

It's not ideal to directly call 'git version --build-options' because
that output goes to stdout. Instead, wrap the version string in a helper
within help.[ch] library, and call that helper from within the bugreport
library.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>