refs: refactor reference status flags

refs: refactor reference status flags

The reference flags encode information like whether or not a reference
is a symbolic reference or whether it may be broken. This information is
stored in a `int flags` bitfield, which is in conflict with our modern
best practices; we tend to use an unsigned integer to store flags.

Change the type of the field to be `unsigned`. While at it, refactor the
individual flags to be part of an `enum` instead of using preprocessor
defines.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>