application/xml mime-type in recent libstdc++ doc changes

[Benjamin De Kosnik]

> > libstdc++-v3/doc/xsl/customization.xsl.in is marked as
> > 
> >   svn:mime-type = application/xml
> > 
> > at least on the 4.7 branch, having some unexpected outcome for svn
> > diff. If this was unintended, could you change the svn:mime-type
> > back to text?

This is unintentional, and frankly I don't know how the inital mime
type was created or how to fix it. Any pointers? Can you do it and tell
me how?

best,
benjamin