git-gui: Paperbag fix blame in subdirectory

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
diff --git a/git-gui.sh b/git-gui.sh
index 62b1a36..2fda4c2 100755
--- a/git-gui.sh
+++ b/git-gui.sh
@@ -1611,7 +1611,7 @@
 	foreach a $argv {
 		if {$is_path || [file exists $_prefix$a]} {
 			if {$path ne {}} usage
-			set path $a
+			set path $_prefix$a
 			break
 		} elseif {$a eq {--}} {
 			if {$path ne {}} {