environment: move access to core.maxTreeDepth into repo settings

environment: move access to core.maxTreeDepth into repo settings

The config setting core.maxTreeDepth is stored in a global variable and
populated by the function git_default_core_config.  This won't work if
we need to access multiple repositories with different values of that
setting in the same process.  Store the setting in struct repo_settings
instead and track it separately for each repository.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>