combo-layer: handle empty commits during "init --history"

combo-layer: handle empty commits during "init --history"

When importing the components during the "combo-layer init" with full
history and relocation into a destination directory, components with
empty commits were not handled because the "mv" command was invoked
with just one parameter.

Replacing that with a find/xargs pair avoids the problem and should
also fix the handling of filenames with special characters (in
particular spaces).

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>