Fix window positions.
Graph and diff windows were swapped if user set `splitbelow` to 1:
    :set splitbelow    :set nosplitbelow
       +---+---+           +---+---+
       | D |   |           | G |   |
       +---+ S |           +---+ S |
       | G |   |           | D |   |
       +---+---+           +---+---+
     D - diff,  G - graph,  S - source
This commit fixes this -- now diff is displayed below graph regardless of
`splitbelow` setting.
    
        | author | Maciej Konieczny <hello@narf.pl> | 
    
        | date | Sun, 22 May 2011 21:54:55 +0200 | 
    
    
        | parents | 321c5d51ee50 | 
    
        | children | 2043d8723096 | 
    
        | branches/tags | (none) | 
    
        | files | plugin/gundo.vim | 
Changes
    
--- a/plugin/gundo.vim	Mon May 09 21:10:50 2011 -0400
+++ b/plugin/gundo.vim	Sun May 22 21:54:55 2011 +0200
@@ -598,11 +598,19 @@
 endfunction"}}}
 
 function! s:GundoOpen()"{{{
+    " Save `splitbelow` value and set it to default to avoid problems with
+    " positioning new windows.
+    let saved_splitbelow = &splitbelow
+    let &splitbelow = 0
+
     call s:GundoOpenPreview()
     exe bufwinnr(g:gundo_target_n) . "wincmd w"
 
     call s:GundoRenderGraph()
     call s:GundoRenderPreview()
+
+    " Restore `splitbelow` value.
+    let &splitbelow = saved_splitbelow
 endfunction"}}}
 
 function! s:GundoToggle()"{{{