Prevent migration changes to info.xml file from affecting selection of version when merging HearThisPacks

[tombogle]

Two changes to try to accomplish this:

1. During data migration, preserve the original file modification time. Note: for most current HT users, the latest migration that modifies this file has probably already happened, so this is not likely to help much.
2. During the merge, omit info.xml from consideration when comparing file date/times. Currently there are only very rare situations (skipping) that this file will be changed independently of recording, so this is probably safe. However, this needs to be reviewed when the changes for HT-359 are merged because that will make it slightly more likely to change the info.xml file alone.

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This change is