Check another error condition in git-mv

When moving multiple files at once, it can happen that
files get the same target name, like in

	git-mv a/foo b/foo destdir

Both a/foo and b/foo target destdir/foo.

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

git-mv.perl

@@ -77,7 +77,7 @@ ()
 
 my (@allfiles,@srcfiles,@dstfiles);
 my $safesrc;
-my %overwritten;
+my (%overwritten, %srcForDst);
 
 $/ = "\0";
 open(F,"-|","git-ls-files","-z")
@@ -123,6 +123,16 @@ ()
 	}
     }
 
+    if ($bad eq "") {
+       if (defined $srcForDst{$dst}) {
+           $bad = "can not move '$src' to '$dst'; already target of ";
+           $bad .= "'".$srcForDst{$dst}."'";
+       }
+       else {
+           $srcForDst{$dst} = $src;
+       }
+    }
+
     if ($bad ne "") {
 	if ($opt_k) {
 	    print "Warning: $bad; skipping\n";