Included patch for persistent undo. Lots of changes and added test.
diff --git a/runtime/doc/undo.txt b/runtime/doc/undo.txt
index 2c8c60c..05e5555 100644
--- a/runtime/doc/undo.txt
+++ b/runtime/doc/undo.txt
@@ -12,7 +12,8 @@
2. Two ways of undo |undo-two-ways|
3. Undo blocks |undo-blocks|
4. Undo branches |undo-branches|
-5. Remarks about undo |undo-remarks|
+5. Undo persistence |undo-persistence|
+6. Remarks about undo |undo-remarks|
==============================================================================
1. Undo and redo commands *undo-commands*
@@ -22,7 +23,7 @@
*:u* *:un* *:undo*
:u[ndo] Undo one change. {Vi: only one level}
-
+ *E830*
:u[ndo] {N} Jump to after change number {N}. See |undo-branches|
for the meaning of {N}. {not in Vi}
@@ -109,6 +110,8 @@
To do the opposite, break a change into two undo blocks, in Insert mode use
CTRL-G u. This is useful if you want an insert command to be undoable in
parts. E.g., for each sentence. |i_CTRL-G_u|
+Setting the value of 'undolevels' also breaks undo. Even when the new value
+is equal to the old value.
==============================================================================
4. Undo branches *undo-branches* *undo-tree*
@@ -201,7 +204,88 @@
while repeating "g-" and "g+" does.
==============================================================================
-5. Remarks about undo *undo-remarks*
+5. Undo persistence *undo-persistence* *persistent-undo*
+
+When unloading a buffer Vim normally destroys the tree of undos created for
+that buffer. By setting the 'undofile' option, Vim will automatically save
+your undo history when you write a file and restore undo history when you edit
+the file again.
+
+The 'undofile' option is checked after writing a file, before the BufWritePost
+autocommands. If you want to control what files to write undo information
+for, you can use a BufWritePre autocommand: >
+ au BufWritePre /tmp/* setlocal noundofile
+
+Vim saves undo trees in a separate undo file, one for each edited file, using
+a simple scheme that maps filesystem paths directly to undo files. Vim will
+detect if an undo file is no longer synchronized with the file it was written
+for (with a hash of the file contents) and ignore it when the file was changed
+after the undo file was written, to prevent corruption.
+
+Undo files are normally saved in the same directory as the file. This can be
+changed with the 'undodir' option.
+
+You can also save and restore undo histories by using ":wundo" and ":rundo"
+respectively:
+ *:wundo* *:rundo*
+:wundo[!] {file}
+ Write undo history to {file}.
+ When {file} exists and it does not look like an undo file
+ (the magic number at the start of the file is wrong), then
+ this fails, unless the ! was added.
+ If it exists and does look like an undo file it is
+ overwritten.
+ {not in Vi}
+
+:rundo {file} Read undo history from {file}.
+ {not in Vi}
+
+You can use these in autocommands to explicitly specify the name of the
+history file. E.g.: >
+
+ au BufReadPost * rundo %:h/UNDO/%:t
+ au BufWritePost * wundo %:h/UNDO/%:t
+
+You should keep 'undofile' off, otherwise you end up with two undo files for
+every write.
+Note: I did not verify this always works!
+
+Note that while reading/writing files and 'undofile' is set most errors will
+be silent, unless 'verbose' is set. With :wundo and :rundo you will get more
+error messages, e.g., when the file cannot be read or written.
+
+NOTE: undo files are never deleted by Vim. You need to delete them yourself.
+
+Reading an existing undo file may fail for several reasons:
+*E822* It cannot be opened, because the file permissions don't allow it.
+*E823* The magic number at the start of the file doesn't match. This usually
+ means it is not an undo file.
+*E824* The version number of the undo file indicates that it's written by a
+ newer version of Vim. You need that newer version to open it. Don't
+ write the buffer if you want to keep the undo info in the file.
+"Undo file contents changed"
+ The file text differs from when the undo file was written. This means
+ the undo file cannot be used, it would corrupt the text.
+*E825* *E826* The undo file does not contain valid contents and cannot be
+ used.
+*E827* The magic number at the end of the file was not found. This usually
+ means the file was truncated.
+
+Writing an undo file may fail for these reasons:
+*E828* The file to be written cannot be created. Perhaps you do not have
+ write permissions in the directory.
+"Will not overwrite with undo file, cannot read"
+ A file exists with the name of the undo file to be written, but it
+ cannot be read. You may want to delete this file or rename it.
+"Will not overwrite, this is not an undo file"
+ A file exists with the name of the undo file to be written, but it
+ does not start with the right magic number. You may want to delete
+ this file or rename it.
+*E829* An error occurred while writing the undo file. You may want to try
+ again.
+
+==============================================================================
+6. Remarks about undo *undo-remarks*
The number of changes that are remembered is set with the 'undolevels' option.
If it is zero, the Vi-compatible way is always used. If it is negative no