vimdiff mergetool diff mode detection in vimrc broken by commit 0041797449

Hi all,

I believe commit 0041797449da61f4131bb1673fa5d4c3af48240d ("vimdiff: new implementation with layout support") broke the ability to detect diff mode in a .vimrc, because it removed '-d' from the command line use to invoke vim ('&diff' evaluates to false, because diffmode isn't enabled until the commands are run, which happens _after_ vimrc is executed AFAICT). I check &diff in my vimrc to set various settings specifically when doing a diff/merge.

Just adding '-d' back breaks the layouts unfortunately. I can hack around this with a heuristic that looks for a mergetool-like invocation, but ideally there would be a way to set the canonical diff setting correctly. It may be as simple as adding some extra commands to close the extra windows opened by adding '-d'.

Thoughts?

Cheers, Andrew