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

"vimdiff3" regression fix. source: <20220810154618.307275-1-felipe.contreras@gmail.com> * 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