symlinks: use unsigned int for flags

symlinks: use unsigned int for flags

The 'flags' and 'track_flags' fields in symlinks.c are used
strictly as a collection of bits (using bitwise operators including
&, |, ~). Using a signed integer for bitmasks may lead to undefined
behavior with shift operations and logic errors if the MSB is touched.

Change these fields from 'int' to 'unsigned int' to match our usage
patterns.

Signed-off-by: Tian Yuchen <a3205153416@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>