[RFC] Fsck while fetching

As the infrastructure for checking the objects and their connectivity
while pushing is in place, it is easy to do the same while fetching on
non shallow repositories.

On shallow repositories this is currently only possible, if the list
of shallow commits does not change (eg. no depth option).

The neccessary changes are:
* Read a config option in fetch-pack to disable it (default: true)
* Add in fetch-pack --strict to the command lines of index-pack and 
  unpack-objects, if the config option is true and args.depth is 0.

This leads to the question:
* How should the config option be named? fetch.fsckObjects? Should
  receive.fsckObjects be reused? Should receive.fsckObjects be renamed
  to a new name, which is used in both cases?

On additional question:
* fsck has a strict option, which looks for files with 0664 mode. How
  should such files be handled with the checking while pushing/fetching:
  ignored, rejected or config option for the behaviour?

mfg Martin Kögler