Use git to calculate trusted mtimes

You can remark at the bottom of each page I provide a last modification date. This label was first calculated using the mtime of the file on the file system. But many times I modify this date just to force some recompilation. Therefore the date wasn’t a date of real modification.

I use git to version my website. And fortunately I can know the last date of real change of a file. This is how I do this with nanoc:

def gitmtime
    filepath=@item.path.sub('/Scratch/','content/html/').sub(/\/$/,'')
    ext=%{.#{@item[:extension]}}
    filepath<<=ext
    if not FileTest.exists?(filepath)
        filepath.sub!(ext,%{#{@item.raw_filename}#{ext}})
    end
    str=`git log -1 --format='%ci' -- #{filepath}`
    if str.nil? or str.empty?
        return Time.now
    else
        return DateTime.parse( str )
    end
end

Of course I know it is really slow and absolutely not optimized. But it works as expected. Now the date you see at the bottom is exactly the date I modified the content of the page.

Edit: Thanks to Eric Sunshine and Kris to provide me some hints at cleaning my code.