Complex schema merge incorrectly flagged as conflict.

Steps to reproduce:

```
git checkout nicktobey/schemarepro
go test libraries/doltcore/merge/schema_merge_test.go -run "^\QTestSchemaMerge\E$/^\Qcolumn_type_change_tests\E$"
```

It seems that dropping a column can affect the tag of other columns in a table. Tags are an internal identifier that we use to recognize columns across branches. If the tag changes, we may perceive the same column in two branches as different and result in a schema conflict.