Patches the built-in Vim runtime files `plugin/manpager.vim` and
`ftplugin/man.vim` to work around tree issues:

1. `nocompatible` is set unconditionally, overriding options in the
vimrc. Fix it to only set it if necessary.
2. `col` fails with `col: write error` when Vim is invoked through
`man` with `$MANPAGER` (but not when starting Vim normally and
running `:Man`). The reason is unknown. Piping through `cat` fixes
the issue.
3. `man` likes to issue a lot of formatting warnings when output is
not a terminal (or when `MAN_KEEP_STDERR` is set) and the default
`shellredir` captures this. The fix is to temporarily set
`shellredir` to ignore `stderr`.

The patch:

for file in plugin/manpager.vim ftplugin/man.vim
do
diff -u $VIMRUNTIME/$file .vim/after/$file
done

--- $VIMRUNTIME/plugin/manpager.vim
+++ .vim/after/plugin/manpager.vim
@@ -5,7 +5,9 @@
command! -nargs=0 MANPAGER call s:ManPager() | delcommand MANPAGER

function! s:ManPager()
- set nocompatible
+ if &compatible
+ set nocompatible
+ endif
if exists('+viminfofile')
set viminfofile=NONE
endif

--- $VIMRUNTIME/ftplugin/man.vim
+++ .vim/after/ftplugin/man.vim
@@ -209,8 +209,11 @@
endif
let env_cmd = s:env_has_u ? 'env -u MANPAGER' : 'env MANPAGER=cat'
let env_cmd .= ' GROFF_NO_SGR=1'
- let man_cmd = env_cmd . ' man ' . s:GetCmdArg(sect, page) . ' | col -b'
+ let man_cmd = env_cmd . ' man ' . s:GetCmdArg(sect, page) . ' | col -b | cat'
+ let old_srr = &shellredir
+ let &shellredir = '>'
silent exec "r !" . man_cmd
+ let &shellredir = old_srr

if unsetwidth
let $MANWIDTH = ''