vimdiff: new implementation with layout support

vimdiff: new implementation with layout support

When running 'git mergetool -t vimdiff', a new configuration option
('mergetool.vimdiff.layout') can now be used to select how the user
wants the different windows, tabs and buffers to be displayed.

If the option is not provided, the layout will be the same one that was
being used before this commit (ie. two rows with LOCAL, BASE and COMMIT
in the top one and MERGED in the bottom one).

The 'vimdiff' variants ('vimdiff{1,2,3}') still work but, because they
represented nothing else than different layouts, are now internally
implemented as a subcase of 'vimdiff' with the corresponding
pre-configured 'layout'.

Again, if you don't set "mergetool.vimdiff.layout" everything will work
the same as before *but* the arguments used to call {n,g,}vim will be
others (even if you don't/shouldn't notice it):

  - git mergetool -t vimdiff

    > Before this commit:
      {n,g,}vim -f -d -c '4wincmd w | wincmd J' $LOCAL $BASE $REMOTE $MERGED

    > After this commit:
      {n,g,}vim -f -c "echo | split | vertical split | 1b | wincmd l | vertical split | 2b | wincmd l | 3b | wincmd j | 4b | tabdo windo diffthis" -c "tabfirst" $LOCAL $BASE $REMOTE $MERGED

  - git mergetool -t vimdiff1

    > Before this commit:
      {n,g,}vim -f -d -c 'echon "..."' $LOCAL $REMOTE

    > After this commit:
      {n,g,}vim -f -c "echo | vertical split | 1b | wincmd l | 3b | tabdo windo diffthis" -c "tabfirst" $LOCAL $BASE $REMOTE $MERGED

  - git mergetool -t vimdiff2

    > Before this commit:
      {n,g,}vim -f -d -c 'wincmd l' $LOCAL $MERGED $REMOTE

    > After this commit:
      {n,g,}vim -f -c "echo | vertical split | 1b | wincmd l | vertical split | 4b | wincmd l | 3b | tabdo windo diffthis" -c "tabfirst" $LOCAL $BASE $REMOTE $MERGED

  - git mergetool -t vimdiff3

    > Before this commit:
      {n,g,}vim -f -d -c 'hid | hid | hid' $LOCAL $REMOTE $BASE $MERGED

    > After this commit:
      {n,g,}vim -f -c "echo | 4b | bufdo diffthis" -c "tabfirst" $LOCAL $BASE $REMOTE $MERGED

Despite being different, I have manually verified that they generate the same
layout as before.

Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>