show-branch.c: abstract out "flags" operation

The current code knows how "flags" are used to record reachability
from each "rev" too much.  Move as much knowledge as possible out of
the main traversal logic about commit->object.flags by introducing
wrapper functions.  We still use the higher bits of flags word for
reachability from revs, but eventually we would want to only use the
UNINTERESTING bit to mark common ancestors we do not need to dig
further, and move per-rev bits into commit slabs.
show-branch.c: remove "all_mask"

It is only used at one place, to mask flags that is further masked
with all_revs, which is a proper subset of all_mask, so the use is a
no-op.

Signed-off-by: Junio C Hamano <gitster@pobox.com>