Merge branch 'jk/fetch-pack-fsck-wo-lock-pack'

"git fetch-pack -k -k" without passing "--lock-pack" (which we
never do ourselves) did not work at all, which has been corrected.

* jk/fetch-pack-fsck-wo-lock-pack:
  fetch-pack: fix segfault when fscking without --lock-pack

diff --cc fetch-pack.c
Simple merge

diff --cc t/t5500-fetch-pack.sh
Simple merge