Fix fc_sort to be deterministic

fc_sort uses its own implementation of merge sort, but it's
unnecessarily complex and sorting criteria isn't clear: it only
compares lengths and existences of fields. So it can give different
results on the same input (same set of entries, different order).

This fixes it so that output is always deterministic, regardless of
the order of lines in input files.

Bug: N/A
Test: try to run fc_sort several times on same input with different
      line orders, and see the results.

Change-Id: I982a35a4ae9e115030a8598027bbf1181ee77a7d
1 file changed