git-gui: Handle gitgui tags in version gen.

I've decided to use gitgui-0.5 as the format for tags in the
git-gui repository.  The prefix of gitgui was chosen here to
make its namespace different from the namespace used by git
itself, allowing developers to pull both tag namespaces into
the same repository.

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

GIT-VERSION-GEN

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 GVF=GIT-VERSION-FILE
-DEF_VER=v0.5.GIT
+DEF_VER=0.5.GIT
 
 LF='
 '
@@ -11,19 +11,17 @@ LF='
 if VN=$(git describe --abbrev=4 HEAD 2>/dev/null) &&
    case "$VN" in
    *$LF*) (exit 1) ;;
-   v[0-9]*) : happy ;;
+   gitgui-[0-9]*) : happy ;;
    esac
 then
-	VN=$(echo "$VN" | sed -e 's/-/./g');
+	VN=$(echo "$VN" | sed -e 's/^gitgui-//;s/-/./g');
 elif test -f version
 then
 	VN=$(cat version) || VN="$DEF_VER"
 else
 	VN="$DEF_VER"
 fi
 
-VN=$(expr "$VN" : v*'\(.*\)')
-
 dirty=$(sh -c 'git diff-index --name-only HEAD' 2>/dev/null) || dirty=
 case "$dirty" in
 '')