xdiff: cast arguments for ctype functions to unsigned char

xdiff: cast arguments for ctype functions to unsigned char

The ctype functions isspace(), isalnum(), et al take an integer
argument representing an unsigned character, or -1 for EOF.  On
platforms with a signed char, it is unsafe to pass a char to them
without casting it to unsigned char first.

Most of git is already shielded against this by the ctype
implementation in git-compat-util.h, but xdiff, which uses libc
ctype.h, ought to be fixed.

Noticed-by: der Mouse <redacted>
Reported-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>