"vimdiff3" regression fix.
* fc/vimdiff-layout-vimdiff3-fix:
mergetools: vimdiff: simplify tabfirst
mergetools: vimdiff: fix single window layouts
mergetools: vimdiff: rework tab logic
mergetools: vimdiff: fix for diffopt
mergetools: vimdiff: silence annoying messages
mergetools: vimdiff: make vimdiff3 actually work
mergetools: vimdiff: fix comment