```
Remove the op== on PartiallySignedTransaction, which only checks that
the CTransactions are equal. Instead, check this directly in Merge, and
return false if the CTransactions are not equal (so the PSBTs cannot be
merged.)
```
Partial backport of core PR14978:
https://github.com/bitcoin/bitcoin/pull/14978/commits/78b9893d020e8b1351565f1adbf591cb32f6dc90