git-gui: Change base version to 0.6.

This is the start of the 0.6 series of git-gui.  I'm calling it 0.6
(rather than any other value) as I already had a private tag on
one system based on 0.5, and that tag is quite a bit behind this
version.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

GIT-VERSION-GEN

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 GVF=GIT-VERSION-FILE
-DEF_VER=0.5.GIT
+DEF_VER=0.6.GITGUI
 
 LF='
 '