git: implement diff manifest method
This makes 'hg diff' work.
from__future__importabsolute_importfrommercurial.i18nimport_frommercurialimportchangegroup,error,extensionsdefabort(orig,*args,**kwargs):raiseerror.Abort(_('this is an exercise'))defuisetup(ui):extensions.wrapfunction(changegroup,'getbundler',abort)