Not scan files on git version < 1.8.5

[killuazhu]

Git version before 1.8.5 does not support -C <path> option.

When running detect-secrets with pre-1.8.5 git version, _get_git_tracked_files would return None since the exception is silently ignored. This causes non of the tracked files been scanned.

git help on 1.8.3

$ git help
usage: git [--version] [--help] [-c name=value]
           [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
           [-p|--paginate|--no-pager] [--no-replace-objects] [--bare]
           [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
           <command> [<args>]

[domanchi]

Added to FAQ. Closing as won't fix.