object.h: fix stale entries in object flag allocation table

object.h: fix stale entries in object flag allocation table

Update three stale entries found during an audit of the flag
allocation table:

 - sha1-name.c was renamed to object-name.c
 - builtin/show-branch.c uses bits 0 and 2-28, not 0-26
   (REV_SHIFT=2, MAX_REVS=FLAG_BITS-REV_SHIFT=27)
 - negotiator/skipping.c uses bits 2-5 like negotiator/default.c
   (ADVERTISED on bit 3 instead of COMMON_REF)

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>