mergetools: add vimdiff3 mode

It's similar to the default, except that the other windows are hidden.
This ensures that removed/added colors are still visible on the main
merge window, but the other windows not visible.

Specially useful with merge.conflictstyle=diff3.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
mergetool: run prompt only if guessed tool

It's annoying to see the prompt:

  Hit return to start merge resolution tool (foo):

Every time the user does 'git mergetool' even if the user already
configured 'foo' as the wanted tool.

Display this prompt only when the user hasn't explicitly configured a
tool.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
merge: enable defaulttoupstream by default

There's no point in this:

% git merge
fatal: No commit specified and merge.defaultToUpstream not set.

We know the most likely scenario is that the user wants to merge the
upstream, and if not, he can set merge.defaultToUpstream to false.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'fc/mergetools-vimdiff3' into next

* fc/mergetools-vimdiff3:
  mergetools: add vimdiff3 mode
Merge branch 'fc/mergetool-prompt' into next

* fc/mergetool-prompt:
  mergetool: run prompt only if guessed tool
Merge branch 'fc/merge-default-to-upstream' into next

* fc/merge-default-to-upstream:
  merge: enable defaulttoupstream by default