Problem: When piping raw manpage content into `:Man!`, buf name is
set to 'man://.. ref', but the check only matches the prefix.
Allows duplicate buffers to be created, triggering E95.
Solution: Match full buf name instead of only 'man://' prefix.
If the buffer already exists, generate a unique name with
'man://' .. 'ref' .. '?new=' format.
Refs: #30132