Add git support

[fxkr]

Right now, diffing two git versions of the same file is really cumbersome (git show $rev1:./foo.brd > old.brd ; git show $rev2:./foo.brd > new.brd ; eagle-diff old.brd new.brd).

[karlp]

Should be just a matter of telling git how to use diff right? like done here: https://github.com/jotux/eagle-diff/blob/master/readme.md#add-this-to-your-gitconfig

[fxkr]

Nice, I didn't know about that.

I guess it'd have to create a temporary directory somewhere, put the images there, display the images somehow (I don't think there's a good portable way to display multiple images), and remove the directory when whatever image viewer was used exits.

[karlp]

so, what I've got working is...

in ~/.gitconfig

[difftool "eagle"]
    cmd = eagle-diff -f -d 100 --eagle /home/karlp/tools/eagle-tastic/bin/eagle $LOCAL $REMOTE

then

git difftool -t eagle rev1 rev2 filename.{brd,sch}

That works. It's not as nice as straight git diff. Using the .gitattributes from the other repo works, but we'd need to make a wrapper script that takes a fixed set of 7 parameters. I couldn't work out a neat way around that.