git-gui: Fix parsing of <rev> <path-which-not-present-in-worktree>

Commit e3d06ca9 (git-gui: Detect full path when parsing arguments) broke

    git gui blame rev path-not-present-in-worktree

in particular this does not work anymore:

    # in linux.git
    $ git gui blame 2bb8c26242c2393b097a993ffe9b003ec9b85395 drivers/net/sky2.c

This patch restores the original functionality in this case.

Acked-by: Andrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

git-gui.sh

@@ -3015,6 +3015,7 @@ blame {
 
 		if {$is_path} {
 			if {$path ne {}} usage
+			set path [normalize_relpath $_prefix$a]
 			break
 		} elseif {$a eq {--}} {
 			if {$path ne {}} {