In commit 63f0ca3263, `tagfunc` was introduced to
`runtime/autoload/man.vim`. Nonetheless the tag function instead
of using a short buffer name (e.g. `man://foo(3)`) uses the full
path to the man page (e.g. `man:///usr/share/.../foo.3.gz`). This
behaviour is inconsistent with `:Man!`, thus this commit.
Closes#13334