difftool: sanitize $workdir as early as possible

difftool: sanitize $workdir as early as possible

The double-slash fixup on the $workdir variable was being
performed just-in-time to avoid double-slashes in symlink
targets, but the rest of the code was silently using paths with
embedded "//" in them.

A recent user-reported error message contained double-slashes.
Eliminate the issue by sanitizing inputs as soon as they arrive.

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