maintenance: add incremental-repack auto condition

maintenance: add incremental-repack auto condition

The incremental-repack task updates the multi-pack-index by deleting pack-
files that have been replaced with new packs, then repacking a batch of
small pack-files into a larger pack-file. This incremental repack is faster
than rewriting all object data, but is slower than some other
maintenance activities.

The 'maintenance.incremental-repack.auto' config option specifies how many
pack-files should exist outside of the multi-pack-index before running
the step. These pack-files could be created by 'git fetch' commands or
by the loose-objects task. The default value is 10.

Setting the option to zero disables the task with the '--auto' option,
and a negative value makes the task run every time.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>