join: fix autoformat

There was an issue with autoformat when the files had a different
number of columns in the first line. This commit fixes the issue and
extends the related test to cover this case.

tests/fixtures/join/autoformat.expected

@@ -1,5 +1,5 @@
-1 a a
-2 b b
-3 c d
-4 d g
-5 e i
+1 a a b
+2 b b c
+3 c d e
+4 d g h
+5 e i 

tests/fixtures/join/different_lengths.txt

@@ -1,4 +1,4 @@
-1 a
+1 a b
 2 b c
 3 d e f
 4 g h