In match_columns, only show ColumnMatches for the current file's columns, not every column the user has ever seen.
Problem introduced in a9a6b4e087caae42250586c8806e3d5b353aecd7.
Fixes #19
Merge pull request #20 from jnm/jnm-fix-match_columns-and-emptiness-test

Fix match_columns and emptiness test