Update bash completion for git-config options

Update bash completion for git-config options

A few new configuration options grew out of the woodwork during the
1.5.2 series.  Most of these are pretty easy to support a completion
of, so we do so.

I wanted to also add completion support for the <driver> part of
merge.<driver>.name but to do that we have to look at all of the
.gitattributes files and guess what the unique set of <driver>
strings would be.  Since this appears to be non-trivial I'm punting
on it at this time.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>