git: split out idx file writing to a separate class

git: split out idx file writing to a separate class

Split the idx file writing into a separate class, to make that
kind of action available separately. This will be useful for the
next patch where we use it to test some idx/midx code.

In the future, it'll also be useful for encrypted repositories
since the idx format there will be useful for local caching to
take advantage of midx and bloom code as is, but the packwriter
will of course not be useful.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>