Incompatible '+=' syntax in git-completion.bash

Hi there,

I noticed a change in commit 734b2f0 on
contrib/completion/git-completion.bash which reverted a syntax fix for
'+=' syntax [1], the syntax does not work for bash < 3.1.  As far as I
know, bash 3.0.x is still widely used on some old servers, could
someone add the fix back again?

Thanks,
Matt

[1] https://github.com/git/git/commit/734b2f0532d847a9f566183982f83ddea8d8d197#commitcomment-3664571