The path list builder had a branch for the case the source is not in index, but
this can happen only if the source was a directory. However, in that case we
have already expanded the list to the directory contents and set mode
to WORKING_DIRECTORY, which is tested earlier.

The patch removes the superfluous branch and adds an assert() instead. git-mv
testsuite still passes.
Signed-off-by: NPetr Baudis <pasky@suse.cz>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>