builtin/mv: refactor `add_slash()` to always return allocated strings

builtin/mv: refactor `add_slash()` to always return allocated strings

The `add_slash()` function will only conditionally return an allocated
string when the passed-in string did not yet have a trailing slash. This
makes the memory ownership harder to track than really necessary.

It's dubious whether this optimization really buys us all that much. The
number of times we execute this function is bounded by the number of
arguments to git-mv(1), so in the typical case we may end up saving an
allocation or two.

Simplify the code to unconditionally return allocated strings.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>