read_raw_ref(): change flags parameter to unsigned int

read_raw_ref(): change flags parameter to unsigned int

read_raw_ref() is going to be part of the vtable for reference backends,
so clean up its interface to use "unsigned int flags" rather than "int
flags". Its caller still uses signed int for its flags arguments. But
changing that would touch a lot of code, so leave it for now.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: David Turner <redacted>
Signed-off-by: Junio C Hamano <redacted>