Documentation quotes commit messages 14 times with double-quotes, and 7
times with single-quotes. The patch turns everything to double-quotes.

A nice side effect is that documentation becomes more Windoze-friendly
as AFAIK single quotes won't work there.
Signed-off-by: NSergei Organov <osv@javad.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>