git-mv: fully detect 'directory moved into itself'

This gives a better error message when trying to move a directory
into some subdirectory of itself; ie. no real bug fix: renaming
already failed before, but with a strange "invalid argument".

Signed-off-by: Josef Weidendorfer <Josef.Weidendorfer@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-mv.perl

@@ -108,7 +108,7 @@ ()
 	}
     }
 
-    if (($bad eq "") && ($src eq $dstDir)) {
+    if (($bad eq "") && ($dst =~ /^$src\//)) {
 	$bad = "can not move directory '$src' into itself";
     }