vimdiff: make layout engine more robust against user vim settings

vimdiff: make layout engine more robust against user vim settings

'vim' has two configuration options ('splitbelow' and 'splitright') that
change the way the 'split' command behaves. When they are set, the
commands that the layout engine generates no longer work as expected.

In order to fix this we can append special keyword 'leftabove' to each
'split' and 'vertical split' subcommand found inside the command string
generated by the layout engine.

This works because whatever comes after 'leftabove' will temporally
ignore settings 'splitbelow' and 'splitright'.

Reported-by: Matthew Klein <mklein994@gmail.com>
Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>