Fix: restore focus to correct window. Add: Diff panel status bar displays how much text changed.
Introduced a better approach to find the target window
In winEnter au, the undotree() returns the content before entering the target window.
Echo error info if requirements not meet.
Only popup error message when user want's to open the undotree window
Makes undotree work with plugins that fold diffs.

Some plugins fold diffs by default.  If such a plugin is installed
along with undotree, `diffpanel.Show()` sets the filetype for
diffpanel to `diff`, which results in a big, happy, closed fold.  Then
`undotree.Draw()` helpfully tries to deletes the last line in the diff
buffer, believing it empty.  Except, said line is now the fold.

The patch addresses this problem by opening the fold, if any.  Perhaps
a better fix would be to make sure the deleted line is actually empty
before removing it.
Merge pull request #15 from lcd047/master

Undotree should work with plugins that fold diffs.
Delete last empty line in diff window only when it's empty
Add hotkey 'q' to close undo panel
add option to control diff command line
Merge pull request #19 from eikevons/master

add option to control diff command line
Updated README
Updated README
Added 4 new layout styles.
Added commands for hiding and showing the undotree

The commands are :UndotreeShow and :UndotreeHide
Merge pull request #26 from AndreasHjortland/master

Added commands for hiding and showing the undotree
Added UndotreeFocus command to jump back from buffer to Undotree window.
Set focus after invoking UndotreeShow (even if it's visible)