cosmetic bug: "git show --pretty:format" produces different output than "git show --format"

Hi!

I noticed that git produces differing output for
"git show --pretty:format show" and "git show --format", even though
the man page says that both are equivalent.
"git show --format" always appends an empty line at the end. The only
way I've found to get rid of that empty line in --format is by adding
-s option.

So, "git show --pretty:format" and "git show -s --format" do produce
the same output.

I can reproduce this issue in Ubuntu Trusty (git version 1.9.1) and
Debian Jessie (git version 2.1.4).