Merge branch 'fc/vimdiff-layout-vimdiff3-fix'

"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