Merge branch 'nd/clone-connectivity-shortcut'

Special case "git clone" and use lighter-weight implementation to
check the completeness of the history behind refs.

* nd/clone-connectivity-shortcut:
clone: open a shortcut for connectivity check
index-pack: remove dead code (it should never happen)
fetch-pack: prepare updated shallow file before fetching the pack
clone: let the user know when check_everything_connected is run

diff --cc builtin/clone.c
Simple merge

diff --cc git.c
Simple merge

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