zsh: complete unquoted paths with spaces correctly

The following is the description of -Q flag of zsh compadd [1]:

This flag instructs the completion code not to quote any
metacharacters in the words when inserting them into the command
line.

Let's say there is a file named 'foo bar.txt' in repository, but it's
not yet added to the repository. Then the following command triggers a
completion:

git add fo<Tab>
git add 'fo<Tab>
git add "fo<Tab>

The completion results in bash:

git add foo\ bar.txt
git add 'foo bar.txt'
git add "foo bar.txt"

While them in zsh:

git add foo bar.txt
git add 'foo bar.txt'
git add "foo bar.txt"

The first one, where the pathname is not enclosed in quotes, should
escape the space with a backslash, just like bash completion does.
Otherwise, this leads git to think there are two files; foo, and
bar.txt.

The main cause of this behavior is __gitcomp_file_direct(). The both
implementions of bash and zsh are called with an argument 'foo bar.txt',
but only bash adds a backslash before a space on command line.

[1]: http://zsh.sourceforge.net/Doc/Release/Completion-Widgets.html

Signed-off-by: Chayoung You <yousbe@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>