builtin/gc.c: guess the size of the revindex

builtin/gc.c: guess the size of the revindex

'estimate_repack_memory()' takes into account the amount of memory
required to load the reverse index in memory by multiplying the assumed
number of objects by the size of the 'revindex_entry' struct.

Prepare for hiding the definition of 'struct revindex_entry' by removing
a 'sizeof()' of that type from outside of pack-revindex.c. Instead,
guess that one off_t and one uint32_t are required per object. Strictly
speaking, this is a worse guess than asking for 'sizeof(struct
revindex_entry)' directly, since the true size of this struct is 16
bytes with padding on the end of the struct in order to align the offset
field.

But, this is an approximation anyway, and it does remove a use of the
'struct revindex_entry' from outside of pack-revindex internals.

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