Add core.mode configuration

So that we can specify general modes of operation, specifically, add the
'next' mode, which makes Git pre v2.0 behave as Git v2.0.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
config: add core.mode = progress pseudo-config

  merge.defaulttoupstream = true
  branch.autosetupmerge = always
  mergetool.prompt = false

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>