`update-drafts` shouldn't modify `draft_date` when there are no other changes

[ddbeck]

Updating draft_date produces lots of noise in the diff on updates. See #1715 for an example. In that case, there are no real changes to the feature files, just updates to the dates.

Instead, the script ought to check if there are any differences except the date before writing a file. If the only difference is the date, then the file ought to be skipped. This will also make it easier to find areas we haven't touched recently.

[autonome]

This also fixes a dev workflow papercut when merging, b/c these always show as conflicts.