git mv: do not keep slash in `git mv dir non-existing-dir/`

git mv: do not keep slash in `git mv dir non-existing-dir/`

When calling `rename("dir", "non-existing-dir/")` on Linux, it silently
succeeds, stripping the trailing slash of the second argument.

This is all good and dandy but this behavior disagrees with the specs at

http://pubs.opengroup.org/onlinepubs/9699919799/functions/rename.html

that state clearly regarding the 2nd parameter (called `new`):

	If the `new` argument does not resolve to an existing directory
	entry for a file of type directory and the `new` argument
	contains at least one non- <slash> character and ends with one
	or more trailing <slash> characters after all symbolic links
	have been processed, `rename()` shall fail.

Of course, we would like `git mv dir non-existing-dir/` to succeed (and
rename the directory "dir" to "non-existing-dir"). Let's be extra
careful to remove the trailing slash in that case.

This lets t7001-mv.sh pass in Bash on Windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>