In order to find if there was already an open man page, the :Man command
would cycle through each window to see if &ft=='man'. This triggers
autocmds, e.g. BufEnter, unnecessarily and can have unexpected
side-effects.
Change the logic to check each window's ft without switching to it
unless it is actually a man window.
Signed-off-by: Joshua Rubin <me@jawa.dev>