path.c: char is not (always) signed

If a "char" in C is signed or unsigned is not specified, because it is
out of tradition "implementation dependent".
Therefore constructs like "if (name[i] < 0)" are not portable,
use "if (name[i] & 0x80)" instead.

Detected by "gcc (Raspbian 6.3.0-18+rpi1+deb9u1) 6.3.0 20170516" when
setting
DEVELOPER = 1
DEVOPTS = extra-all

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -1368,7 +1368,7 @@ only_spaces_and_periods:
 			saw_tilde = 1;
 		} else if (i >= 6)
 			return 0;
-		else if (name[i] < 0) {
+		else if (name[i] & 0x80) {
 			/*
 			 * We know our needles contain only ASCII, so we clamp
 			 * here to make the results of tolower() sane.