maintenance: add loose-objects.batchSize config

maintenance: add loose-objects.batchSize config

The 'loose-objects' task of 'git maintenance run' first deletes loose
objects that exit within packfiles and then collects loose objects into
a packfile. This second step uses an implicit limit of fifty thousand
that cannot be modified by users.

Add a new config option that allows this limit to be adjusted or ignored
entirely.

While creating tests for this option, I noticed that actually there was
an off-by-one error due to the strict comparison in the limit check. I
considered making the limit check turn true on equality, but instead I
thought to use INT_MAX as a "no limit" barrier which should mean it's
never possible to hit the limit. Thus, a new decrement to the limit is
provided if the value is positive. (The restriction to positive values
is to avoid underflow if INT_MIN is configured.)

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