Make bob builds show the full source git commit hash in buildinfo.
The Windows binaries, and both Windows and Unix source archives,
output from a bob build will now include the full SHA-1 of the source
git commit in their buildinfo (hence in all the About boxes and
command-line version output).
This will be occasionally useful to me at release time (there was that
one embarrassing incident where I managed not to notice that I'd made
a release build from entirely the wrong commit), but mostly, it just
seems like an obviously useful thing to put in a general buildinfo
section now that there is one.