alternates: use fspathcmp to detect duplicates

alternates: use fspathcmp to detect duplicates

On a case-insensitive filesystem, we should realize that
"a/objects" and "A/objects" are the same path. We already
use fspathcmp() to check against the main object directory,
but until recently we couldn't use it for comparing against
other alternates (because their paths were not
NUL-terminated strings). But now we can, so let's do so.

Note that we also need to adjust count-objects to load the
config, so that it can see the setting of core.ignorecase
(this is required by the test, but is also a general bugfix
for users of count-objects).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>