difftool: create a tmpdir path without repeated slashes

difftool: create a tmpdir path without repeated slashes

The paths generated by difftool are passed to user-facing diff tools.
Using paths with repeated slashes in them is a cosmetic blemish that
is exposed to users and can be avoided.

Use a strbuf to create the buffer used for the dir-diff tmpdir.
Strip trailing slashes from the value read from TMPDIR to avoid
repeated slashes in the generated paths.

Adjust the error handling to avoid leaking strbufs and to avoid
returning -1 to cmd_main().

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>