[BUG] assertion failure in builtin-mv.c with "git mv -k"

Hi,

Just found a bug in builtin-mv.c. Here's a script to reproduce:

mkdir git cd git git init touch controled git add controled && git commit -m "init" touch foo1 foo2 mkdir dir git mv -k foo* dir/

The output is the following:

Initialized empty Git repository in /tmp/git/.git/
[master (root-commit)]: created 694563b: "init"
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 controled
git: builtin-mv.c:216: cmd_mv: Assertion `pos >= 0' failed.
./bug.sh: line 10: 12919 Aborted                 git mv -k foo* dir/

Apparently, this happens when using "git mv -k" with more than one unversionned file. The code ignores the first one, but still goes through this

	for (i = 0; i < argc; i++) {
		const char *src = source[i], *dst = destination[i];
		enum update_mode mode = modes[i];
		int pos;
		if (show_only || verbose)
			printf("Renaming %s to %s\n", src, dst);
		if (!show_only && mode != INDEX &&
				rename(src, dst) < 0 && !ignore_errors)
			die ("renaming %s failed: %s", src, strerror(errno));

		if (mode == WORKING_DIRECTORY)
			continue;

		pos = cache_name_pos(src, strlen(src));
		assert(pos >= 0); /* <----- this is the one */
		if (!show_only)
			rename_cache_entry_at(pos, dst);
	}

for the second, and it crashes on the assertion (gdb says "pos" here is an unversionned file name).

If anyone has time to fix this ...

Thanks,