scalar: address -Wsign-compare warnings

scalar: address -Wsign-compare warnings

There are two -Wsign-compare warnings in "scalar.c", both of which are
trivial:

  - We mistakenly use a signed integer to loop towards an upper unsigned
    bound in `cmd_reconfigure()`.

  - We subtract `path_sep - enlistment->buf`, which results in a signed
    integer, and use the value in a ternary expression where second
    value is unsigned. But as `path_sep` is being assigned the result of
    `find_last_dir_sep(enlistment->buf + offset)` we know that it must
    always be bigger than or equal to `enlistment->buf`, and thus the
    result will be positive.

Address both of these warnings.

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