Add user_ftplugin.vim and user_indent.vim plugins

This reverts commit 09b83b6 and replaces it with a working version.

Because of the order in which the autocmd hooks run, the attempted
method of adding unloading instructions for my custom ftplugin and
indent rules to the b:undo_ftplugin and b:undo_indent doesn't actually
work.

This is because the custom rules for both groups from ~/.vim are sourced
*first*, before their core versions, so the changes the custom rules
made to b:undo_ftplugin and b:undo_indent are simply clobbered by the
core version when it loads itself.

Therefore we need to arrange for two things:

1.  A custom variable needs to be checked and executed when the filetype
    changes to revert the changes for the custom ftplugin or indent
    rules.
2.  That execution needs to take place *first* when the filetype
    changes.

I wrote two simple plugins with very similar code that are designed to
run as a user's custom ftplugin.vim and indent.vim implementations,
running before their brethren in the Vim core, and setting up an autocmd
hook to :execute b:undo_user_ftplugin and b:undo_user_indent plugin
respectively.

This seemed to work well, so I've implemented it. It involves adding a
shim to ~/.vim/indent.vim and ~/.vim/ftplugin.vim to "preload" the
plugin when the `filetype indent plugin on` call is made. I've added
that to the relevant Makefile targets.

2 files changed, 48 insertions, 0 deletions

diff --git a/vim/plugin/user_ftplugin.vim b/vim/plugin/user_ftplugin.vim
new file mode 100644
index 00000000..d9739bda
--- /dev/null
+++ b/vim/plugin/user_ftplugin.vim
@@ -0,0 +1,24 @@
+"
+" user_ftplugin.vim: When switching filetypes, look for a b:undo_user_ftplugin
+" variable and use it in much the same way the core's ftplugin.vim does
+" b:undo_ftplugin in Vim >= 7.0. This allows you to undo your own ftplugin
+" files the same way you can the core ones.
+"
+if exists('g:loaded_user_ftplugin')
+      \ || !has('autocmd')
+      \ || &compatible
+  finish
+endif
+let g:loaded_user_ftplugin = 1
+
+function! s:LoadUserFtplugin()
+  if exists('b:undo_user_ftplugin')
+    execute b:undo_user_ftplugin
+    unlet b:undo_user_ftplugin
+  endif
+endfunction
+
+augroup user_ftplugin
+  autocmd!
+  autocmd FileType * call s:LoadUserFtplugin()
+augroup END
diff --git a/vim/plugin/user_indent.vim b/vim/plugin/user_indent.vim
new file mode 100644
index 00000000..01596bdb
--- /dev/null
+++ b/vim/plugin/user_indent.vim
@@ -0,0 +1,24 @@
+"
+" user_indent.vim: When switching filetypes, look for a b:undo_user_indent
+" variable and use it in much the same way the core's indent.vim does
+" b:undo_indent in Vim >= 7.0. This allows you to undo your own indent files
+" the same way you can the core ones.
+"
+if exists('g:loaded_user_indent')
+      \ || !has('autocmd')
+      \ || &compatible
+  finish
+endif
+let g:loaded_user_indent = 1
+
+function! s:LoadUserIndent()
+  if exists('b:undo_user_indent')
+    execute b:undo_user_indent
+    unlet b:undo_user_indent
+  endif
+endfunction
+
+augroup user_indent
+  autocmd!
+  autocmd FileType * call s:LoadUserIndent()
+augroup END