pack-revindex: remove unused 'find_revindex_position()'

pack-revindex: remove unused 'find_revindex_position()'

Now that all 'find_revindex_position()' callers have been removed (and
converted to the more descriptive 'offset_to_pack_pos()'), it is almost
safe to get rid of 'find_revindex_position()' entirely. Almost, except
for the fact that 'offset_to_pack_pos()' calls
'find_revindex_position()'.

Inline 'find_revindex_position()' into 'offset_to_pack_pos()', and
then remove 'find_revindex_position()' entirely.

This is a straightforward refactoring with one minor snag.
'offset_to_pack_pos()' used to load the index before calling
'find_revindex_position()'. That means that by the time
'find_revindex_position()' starts executing, 'p->num_objects' can be
safely read. After inlining, be careful to not read 'p->num_objects'
until _after_ 'load_pack_revindex()' (which loads the index as a
side-effect) has been called.

Another small fix that is included is converting the upper- and
lower-bounds to be unsigned's instead of ints. This dates back to
92e5c77c37 (revindex: export new APIs, 2013-10-24)--ironically, the last
time we introduced new APIs here--but this unifies the types.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>