Merge branch 'jn/clone-add-empty-config-from-command-line'

"git clone --config var=val" is a way to populate the
per-repository configuration file of the new repository, but it did
not work well when val is an empty string.  This has been fixed.

* jn/clone-add-empty-config-from-command-line:
  clone: handle empty config values in -c

diff --cc builtin/clone.c
Simple merge