t7900-subtree: test the "space in a subdirectory name" case

In common case there can be spaces in a subdirectory name. Change tests
accorgingly to this statement.

Also, as far as a call to the `rejoin_msg` function (in `cmd_split`)
does not take into account such a case this patch fixes commit message
when `--rejoin` option is set .

Besides, as `fixnl` and `multiline` functions did not take into account
the "new" tested "space in a subdirectory name" case they become unused
and redundant, so they are removed.

Signed-off-by: Alexey Shumkin <Alex.Crezoff@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/git-subtree.sh

@@ -648,7 +648,7 @@ cmd_split()
 		debug "Merging split branch into HEAD..."
 		latest_old=$(cache_get latest_old)
 		git merge -s ours \
-			-m "$(rejoin_msg $dir $latest_old $latest_new)" \
+			-m "$(rejoin_msg "$dir" $latest_old $latest_new)" \
 			$latest_new >&2 || exit $?
 	fi
 	if [ -n "$branch" ]; then