read-cache: regenerate shared index if necessary

read-cache: regenerate shared index if necessary

When writing a new split-index and there is a big number of cache
entries in the split-index compared to the shared index, it is a
good idea to regenerate the shared index.

By default when the ratio reaches 20%, we will push back all
the entries from the split-index into a new shared index file
instead of just creating a new split-index file.

The threshold can be configured using the
"splitIndex.maxPercentChange" config variable.

We need to adjust the existing tests in t1700 by setting
"splitIndex.maxPercentChange" to 100 at the beginning of t1700,
as the existing tests are assuming that the shared index is
regenerated only when `git update-index --split-index` is used.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>