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.

1 files changed, 27 insertions, 0 deletions

diff --git a/vim/doc/user_indent.txt b/vim/doc/user_indent.txt
new file mode 100644
index 00000000..ff71d575
--- /dev/null
+++ b/vim/doc/user_indent.txt
@@ -0,0 +1,27 @@
+*user_indent.txt*  "Undo" for local indent files
+
+Author:  Tom Ryder <tom@sanctum.geek.nz>
+License: Same terms as Vim itself (see |license|)
+
+This plugin adds an |autocmd| hook to |FileType| to run before the one that the
+core indent.vim sets, to allow setting a b:undo_user_indent variable with
+code to |:execute| when the filetype is next changed, in much the same way that
+the core files in Vim >= 7.0x support a b:|undo_indent| variable.
+
+This will only work if it's loaded *before* indent.vim so that the autocmd
+hooks run in the right order. There are a couple of ways to do this:
+
+1.  Create your own ~/.vim/indent.vim with the following contents:
+
+        silent! runtime plugin/user_indent.vim
+
+    This will then be sourced before the core indent.vim runs. This is the
+    suggested method.
+
+2.  Run the same line in your ~/.vimrc before your `filetype indent on` line.
+
+See also: user_ftplugin.vim.
+
+This plugin lives in Tom Ryder's "dotfiles" suite, and will eventually be spun
+off into a separate distribution as it solidifies and this documentation
+improves.