fsck incoming objects on pull

Since objects are shared between backed up repositories, it is important
to make sure we do not pull a broken object once, thus programming
future corruption of that object after restore in all repositories which
use it.

Object corruption could happen for two reasons:

    - plain storage corruption, or
    - someone intentionally pushing corrupted object with known sha1 to
      any repository.

Second case is even more dangerous, as it potentially allows attacker to
change data in not-available-to-him repositories.

Now objects are checked on pull, and if corrupt, git-backup complains,
e.g. this way:

    RuntimeError: git -c fetch.fsckObjects=true fetch --no-tags ../D/corrupt.git refs/*:refs/backup/20151014-1914/aaa/corrupt.git/*
    error: inflate: data stream error (incorrect data check)
    fatal: loose object 52baccfe8479b61c2a0d5447bc0a6bf7c6827c60 (stored in ./objects/52/baccfe8479b61c2a0d5447bc0a6bf7c6827c60) is corrupt
    fatal: The remote end hung up unexpectedly

git-backup

@@ -455,14 +455,16 @@ def cmd_pull_(pullspecv):
 
                 info('# git  %s\t<- %s' % (prefix, gitrepo))
                 # NOTE --no-tags : do not try to autoextend commit -> covering tag
-                xgit('fetch', '--no-tags', gitrepo,
+                # NOTE fetch.fsckObjects=true : check objects for corruption as they are fetched
+                xgit('-c', 'fetch.fsckObjects=true',
+                     'fetch', '--no-tags', gitrepo,
                         'refs/*:%s%s/*' % (backup_refs_work,
                             # NOTE repo name is quoted as it can contain spaces, and refs must not
                             quote(reprefix(dir_, prefix, gitrepo))),
                             # TODO do not show which ref we pulled - show only pack transfer progress
                             stderr=gitprogress())
 
-                # XXX do we want to fsck source git repo on pull ?
+                # XXX do we want to do full fsck of source git repo on pull as well ?
 
 
             # do not recurse into dirs so marked