[1.8.0] Support quoting in .gitattributes, .gitignore

Proposal:

There's already a patch for .gitattributes [1].

To summarize: .gitattributes separates pattern and attributes by
spaces, therefore spaces could not be present in patterns. There are
tricks to overcome (e.g. substitute spaces with dots) but it's well..
tricky. The patch adds support for quoting full patterns.

While .gitignore does not have the same problem, it'd be nice to
support quoting too for a few reasons:

 - Keep differences between gitattr and gitignore to minimum. A
hero(ine) might come and unify them some day. Don't make his/her work
harder.
 - Is ability to put \n in patterns counted as a good point?

Risks:

Current patterns with leading double quote mark will be
misinterpreted. Though chances are really small.

Migration plan:

I think a release note mentioning this is enough. No migration needed.

But to be safe, we can make post-1.7.5 warn users about patterns with
leading double quote. By 1.8.0, the new behavior will be used.

[1] http://article.gmane.org/gmane.comp.version-control.git/160867
-- 
Duy