declare 1-bit bitfields to be unsigned

declare 1-bit bitfields to be unsigned

As "gcc -pedantic" notices, a two's complement 1-bit signed integer
cannot represent the value '1'.

 dir.c: In function 'init_pathspec':
 dir.c:1291:4: warning: overflow in implicit constant conversion [-Woverflow]

In the spirit of v1.7.1-rc1~10 (2010-04-06), 'unsigned' is what was
intended, so let's make the flags unsigned.

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>