Refactoring for CommitTransactionCommand()/AbortCurrentTransaction()

Refactoring for CommitTransactionCommand()/AbortCurrentTransaction()

fefd9a3fed turned tail recursion of CommitTransactionCommand() and
AbortCurrentTransaction() into iteration.  However, it splits the handling of
cases between different functions.

This commit puts the handling of all the cases into
AbortCurrentTransactionInternal() and CommitTransactionCommandInternal().
Now CommitTransactionCommand() and AbortCurrentTransaction() are just doing
the repeated calls of internal functions.

Reported-by: Andres Freund
Discussion: https://postgr.es/m/20240415224834.w6piwtefskoh32mv%40awork3.anarazel.de
Author: Andres Freund